I'm using Microsoft Security Essentials and it seems to work flawlessly and I also have Malwarebytes running in the background.
Nod32 V4 Another one for Nod32 v4. Installed on Ultimate x86 with no problems, its as stable as it was under Vista 32bit.
I am using AVG Free 8.5 Earlier i also used Avast Pro, but it expired and i don't want to use it anymore, antivirus reviews 2009 website places it after first 10. i also have a free license for Trend Micro 2009 (from my online bank) but a year ago i tried it and it is a bit heavy on resources, and second not compatible with Windows 7...