This is a link to a page by Martin Lewis Money Saving Expert, which gives a list of free programmes which will scan your computer for viruses. The programmes are from safe sources, and are free to download and run. I tried the microsoft one, and it found no errors on my PC.
If you have anti-virus software on your PC which is up to date, you should be fine.
You may need to know what version of Windows you have on your PC in order to download the right version of the virus scan program. To find out what version of windows you have, click on Start, then right click on Computer, and click on Properties.
(For Windows 8, on the Start screen type “windows version” then choose “See if you have a 32-bit or 64-bit version of windows”