Worst case scinario is it needs an operating system reload. If there is nothing you really care about on it just take the restore disc that came with it or the operating system disc (whatever one you have) stick it in and re-boot. Let it do its thing and wipe the hard drive.

Simple, and its free! Then load a virus scanning program on your computer! (I reccomend AVG as a free option myself)

Make sure you scan the computer before it goes online to do anything but download AVG, as the virus can still be resident on the hard drive after a re-load if its one of the nastier varieties.

Hopefully it didn't destroy your hard drive. If it did though, smaller used hard drives at computer shops only run about 50 cents a gig so you can get one that works decently for about $20-$40, maybe less if your town has a computer recycling center.