Sounds to me that you need to do two things initially (i would). check your hard drive fragmentation status (u may b surprised) and do a scan disk (checking both boxes when they come up as you schedule the scan.)
Both these can help computer performance; speed and error wise.
Its doesnt sound like a virus problem.
also try adaware www.lavasoft
as its the most reliable program (been heavily tested and proven by www.download.com
If you really
want to make sure your PC is virus free, ther are two things:
1. Run windows in safe mode (press F8 or F10 after the first startup screen disappears) and run your updated
virus patch program; this mode enables even 'unseen' viruses to be found and deleted.
2. Run an online virus scanner (they always have the latest virus patches).