I'm using AVG on one test system and Avira on the other. Both seem to be working fine so far, except for a few issues with the Security Center plugin for AVG at the start. A day later there was an update that fixed it, so it should be fine now. The _Official_ forum for Windows 7 security is located over on Microsoft's Technet social section. It is located here: http://tinyurl.com/llvgd4
+1 for Avira. Don't know why people are still using AVG(Free, which is based off of the same engine as the paid version), it isn't that great any more. I switched to Aviria after it found a few things AVG missed.
AVG actually tried to tell me NetBeans and .NET updates (Java IDE, downloaded from www.netbeans.org and .NET updates via Windows Update) was a virus! lol. It also told me that my host file (Located in: C:\WINDOWS\system32\drivers\etc) was a virus, which wasn't.
Best Free Anti-Virus software: There was no contest with the freebie –Antivir was above and beyond its competition. Enough said.
One of the antivirus programs being offered is from McAfee which I have been using with no problem. If you are interested you can download it free from there website. It is in the Beta section. Of course all the antivirus I am familiar with at this time is Beta for Windows 7 RC.