Removing Computer Viruses (Advanced)

Not all viruses are spotted with a quick antivirus check, whilst some PCs may behave as if they have a virus when they don't. So how do you deal with these cases?

On this page are several advanced steps you can run through to look for viruses, including a few tips for ruling out a slow computer.

Run through these steps in order, and once you've resolved the issue then just stop - there's no need to work through to the end.

Step 1 - Do some housekeeping

If your antivirus software failed to find any viruses during a quick virus scan, and your computer is still running slowly, then it's worth doing a bit of housekeeping to rule out other common causes. A useful & popular utility for helping here is "CCleaner".

a) Download CCleaner

Click this link (or this one for Mac computers) to download the tool (the Free version is all that we need). Follow the instructions to install and then launch it.

b) Analyze your computer

The CCleaner interface

CCleaner's interface after launching it.

The interface is very simple; once it's loaded make sure that the big "Custom Clean" button on the left (with the icon of a sweeping broom) is selected (not the "Quick Clean" button which is often selected by default).

Down the left you'll also see a list of check boxes grouped under headings such as "Microsoft Edge" or "System". To be really thorough you can go down these and check every box, although be aware that this will reset various items (such as any shortcuts).

Now click the "Analyze" button (at the bottom). The program will take a few seconds whilst it analyses your computer, and will shortly list a range of files that you don’t use and don’t need. These will be grouped by relevant program (for example all the unnecessary "Temp" files that your internet browsers accumulate).

c) Now clear out that junk!

The CCleaner interface after analysing

CCleaner lists its findings after a scan; click "Run cleaner" when you're ready.

You can either simply opt to delete everything at once by clicking "Run Cleaner" or, if you're more advanced, you can go through each of the findings individually by right clicking each to see the options available.

If you do opt to delete everything at once (most people do) then when you click to run the cleaner you'll see a prompt about permanently deleting files, just click OK.

If you have a Mac computer running macOS then skip straight to Step 2; step 1d here is only valid for PCs running Microsoft Windows

d) Spring clean the registry


There is always a slight risk of causing damage to your computer when doing anything to the registry. If your computer isn’t running slowly or exhibiting other problems then you may be safer simply skipping this section.

The second clean-up exercise is to tidy up your computer’s internal system - the "Registry". This is a fundamental part of your computer and how it operates, but over time can become slightly corrupt. Refreshing this can sometimes help your computer to run more efficiently.

The CCleaner interface after analysing

The button for Registry scanning

Click the "Registry" icon on the left of the screen and then "Scan for issues" at the bottom. After a few seconds CCleaner will have listed all the issues it’s found (don’t panic, it’s not uncommon for there to be hundreds of issues!). To fix these click "Fix selected issues".

The CCleaner interface after analysing

A list of issues found after a scan

You will first be prompted to see if you want to create a backup of the existing registry settings. This is always a good idea as the Registry is fundamental to the workings of your computer - an error here (however unlikely) can cause huge problems to your PC. The backup file size is tiny; simply save it somewhere safe that you can remember.

After taking the backup CCleaner will take you through each issue one by one. You can opt to fix or skip each issue individually, or select "Fix all selected issues" to fix them all.

Step 2 - Check what programs run at start up


This step needs some technical knowledge to be able to identify the software that is running; if you're not sure then skip this step.

Another cause of computers running slowly can be other programs running silently in the background without you realising.

Fixing this requires some technical knowledge to know which programs are essential, which are useful, and which can be deleted.

If you’re not very technical or confident with Microsoft Windows then skip this whole step until you can find a suitable friend to help (it's time to bribe your friends with drink or chocolates!).

First we need to bring up the list of programs that launch when we start our computer:

The start-up list in Windows 8 and 10

The start-up list in Windows 8 & 10

The challenge then is to go through these programs and identify those which you don’t need to launch straight away.

Many are not obviously named so be careful you’re not disabling something you need - run a Google search if you’re not sure what a particular program is or does (eg you don’t want to be accidentally disabling your antivirus program!).

To disable a program:

  • In Windows 8 & 10, simply highlight the application and click the "Disable" button in the bottom right of the menu.
  • In Windows 7, deselect the check box next to the application.
  • In macOS (ie Mac computers), highlight the relevant application and click the "-" symbol that's below the list to the left.

Windows 8 & Windows 10 make it a bit easier to choose what to disable because of the "Startup impact" column. This helps identify those programs that have a particularly high impact on the speed of our computer and which we may want to prioritise disabling.

At this point it can be a good idea to restart your computer, to allow all the changes made so far to take full effect.

Step 3 - Run a full virus scan

Run the scan

If the initial virus scan didn’t find anything, and if your PC is still running slow and exhibiting signs of an infection, then re-run the virus scan - but this time select a "Full scan" (or something with a similar name, depending on how your antivirus program describes it).

If you downloaded one of the free virus scanning tools listed on the previous virus help page then you may not have this full scan option; you may need to buy a full antivirus tool instead. Popular ones include those from:

If you want more help finding the right product then see our guide to Choosing Antivirus Software.

Running a full scan will search your computer in much more depth than the quick scan, and may pick up any particularly sneaky viruses that are trying to remain hidden. A full scan, depending on your computer, can take anywhere from between half an hour to several hours, so allow plenty of time for this.

Follow the advice on the previous virus help page for interpreting and dealing with the results of the scan once it completes.

Step 4 - Launch a bootable scan

If you've got this far and think your computer is still infected then wow, you've got a stubborn virus! It's at this point that you'll want to try using a "bootable" virus scan.

This scan is one that runs from a USB stick or CD, and launches on startup before Windows has even loaded. This gives it a much better chance of finding the virus - the virus can't just hide itself amongst all the other noise of Windows.

To do a bootable scan you'll need access to a second computer to prepare it. Follow any of the links below & download the file (they're free), then save it to a USB stick or write it to CD.

Generally you'll then simply switch your computer on with the USB or CD already in it, however the links below all include full details on how to run them (sometimes there's a few steps you'll need to do first in order to boot from a CD or USB):

Step 5 - Get a specialist to look over your PC

If all of the above fails then you’ve been very unlucky! Some viruses are just very stubborn and common techniques for removing them don't work.

If you’ve got to this point it's now worth seeking the help of any technical friends or a computer repair shop. They'll be able to do more advanced checks to determine whether your computer really does have a virus or whether the issues you're seeing are cause by something else.

Whatever the issue they'll hopefully be able to fix it, but even if they can't then don’t panic - you won’t need to buy a new computer! Re-installing Windows (or macOS for Macs) will always work and remove any infection. This can be a hassle and take several hours though so it's always a last resort (and remember to take backups of your personal files first!).

Step 6 - Recovery

If any specific recovery actions are needed to fully remove the virus from your PC then your antivirus software should guide you through those. Regardless of these though, there's a few extra steps that are worth following to fully recover - even after the virus has been cleaned.

Update your computer software

Viruses manage to take hold on your computer by exploiting weaknesses in your software & applications. As manufacturers find or hear about these weaknesses they regularly release updates to fix them - you should always try to keep everything up to date to help prevent future virus infections.

See how to do this with our guide to keeping computer software up to date.

Check your browser settings

If you've noticed strange behaviour when browsing the web, such as being redirected to unusual websites or search engines, then you may want to reset your browser back to its default state.

Some viruses try to change your browser's settings to direct you to pages that are under the virus writers' control - even after you've removed the virus. This might just be to make money from advertising, or it could be an attempt to re-infect your PC.

Click on your favourite web browser below for instructions on how to reset it:

Consider changing passwords

Change your passwords

Some viruses are designed to steal passwords from your computer, either capturing them as you type or by grabbing them from your browser's store.

Because of this it can be a good idea to change your passwords after a virus infection, especially for your most important accounts (such as your bank, social media, and email accounts).

If you know that some of your accounts were directly affected - for example if your friends were receiving spam from your email account - then you should definitely change those ones.

Don't change passwords by visiting websites from your computer if you think it might still be infected; use a friend's computer if yours is taking time to clean. See our guide for changing your passwords safely.

Re-run a virus scan after a few days

Re-run your virus scan after a few days

Antivirus products are great but they don't always catch everything, especially if the virus is new and hasn't been seen before.

For this reason - whatever the result of the initial virus scan - it's always a good idea to run another scan on your computer after a few days when the antivirus developers are likely to know much more of what to look for. Start with Step 3 again (above), ensuring first that your antivirus program is up to date.

Have any feedback on this page? Let us know -