Since many here use an MS operating system and given that, I hope, protecting our systems is near and dear to our hearts, I'm posting a link to the release version of security essentials.
http://www.microsoft.com/security_essentials/
It has a good rep through testing, but will now have to undergo the crucible of release. I am running it on my 64-bit Vista machine but am not turning of my other security tools.
Use it or not, your call.
btw, the pay version of AVG has been doing great for me.