Do you know if your PC has a virus? Do you have an up to date virus scanner insalled on your PC? If not try a free one such as AVG.
One other thing to try is if you are running Windows XP use the system restore function. Go back to a date when you know your machine was running correctly and try restoring your system.
If that fails do you have a backup CD/ DVD of your machine such as Acronis Trueimage or the recovery CD that came with your machine?