I have a really bad computer virus. Ho do I get rid of it?

I bought norton internet security 2010 and I ran it on my computers. One turned out just fine the other had a bunch of stuff to get rid of. After getting rid of every thing I could there was one thing left I could not get rid of as hard as I tried. It is atapi.sys virus. I really need some advice as to how I can get rid of it. It was suggested that I restart my computer turn off the restore system, update my antivirus system, and do a full scan. That didn't work because my computer (which is running on windows xp by the way) wouldn't allow me to check the box to turn off restore system. I could really use some good, no great advice right now! Thanks!!!