Guide Clifford's guide to computer security and maintenance.

Haven't seen it mentioned, but I use Avira AntiVir as my antivirus. Switched from AVG, was having major virus problems, maybe they've fixed their program since then, but if it's the same as it was about a year and a half ago, I wouldn't touch it.

I second Shay's suggestion of MalwareBytes, amazing program. I also use Defraggler, by the same company that makes Ccleaner. All it is is a disk defragging program, but it's a lot better than the standard one that's included with Windows.