AVG Anti-Virus
My personal philosophy when it comes to anti-viruses is, so long as you follow a few simple rules they shouldn't be necessary. Most of the big name commercial anti-virus packages are what I call "bloat-ware". They slow down even the newest computers and in my experience either lull people into a false sense of security or bombard their owners with too many "false positives". As a result I actually don't load them on my personal computers as a general rule.