Best of the Free Anti-virus Choices?

iamjoltman writes "I've been looking to replace the McAfee anti-virus on my parent's XP machine. So, I've been looking at the three free anti-virus choices, AVG Free Edition, avast! Home Edition and AntiVir Personal Edition. I know there are other options, but I believe any others are only on-demand scanners, and that's not an option. So, what does the Slashdot crowd think is the best of these choices? Keep in mind, I'm only looking in anti-virus, I'll go elsewhere for firewall or malware protection."