oh this is easy to fix ... dump XP & install Linux, or buy a MAC ... neither of them need any AV programs at all. A 6lb lump hammer works too.
OK, on a more serious note ... I don't think you have a virus.
The word "virus" is, unfortunately being used to mean ANY infestation you may have contracted. The AV programs you may have been using, are incapable of detecting much in the way of "nasties" in the first place.
You NEED better security, and a different outlook on what "PC security" actually is.
Viruses are not the only nasties, there are others called ... Trojans, Worms, Malware & spyware ... all designed, unlike a virus, to take some control over your PC and use it for their own means. Hence the slowdown.
Two of the best "AV" programs, that also detect other "nasties" have already been mentioned ... they both have fully working trials you can download, they are not free ... Try Anti-Vir Personal if you want a free AV program.
Now, the above programs are a start, however, we need to supliment these with more dedicated programs for the other nasties we WILL incounter on our travels through the Internet.
You will need, at least, to also install ... SpywareBlaster, Spybot Search & Destroy & A-Squared ... and to help "tydie up" after any scan, install CrapCleaner.
All these "programs" NEED to be updated & run on a regular bases ... or you may as wll go learn & install Linux
Your PC maybe so full of crap that a clean re-install wiping the HD may be a better idea.