|The removal of viruses can sometimes affect your PC and its functions that give the appearance of it slowing down when some things are just flat out not working. |
An example is a root virus, which affects your hard drive clusters.. you remove those the wrong way and you'll have to re-install your operating system.
Although I agree with some of the things the previous user said, I do not agree with formatting being the next option, thats your last resort. One thing he did not mention was to clean your registry.. look into a program called Registry Mechanic.
I hope this helps!