I use AVG from Grisoft on my laptop, and a portable version of Clamwin on my flash disk, I recommend AVG anyday, with Clamwin as a possible back option.
Microsoft Security Essentials has been working fine for me. I don't even notice it's there, but it will alert me if I go to a suspicious website (one that contains actual malware) or open a file with a virus. It updates itself everyday without user interaction, too.
. No one mentioned Clamwin? It's light, opensource and you can even run the linux version and have it scan your Windows Partition for threats. http://www.clamwin.com/ ***edit* Ok, it sucks for windows but great for linux I just restored an image after a Rogue/super fake Antivirus hijacked my Windows XP system. Guess Clamwin didn't protect me Yay.
