COMODO ANTIVIRUS works 100% and best of all it's free.
I highly recomend giving it a try you will LOVE it!
I opted not to install the firewall during installation however. Not sure if that plays nice with Windows 7.
+1 on COMODO... put it on 4 installations of Win7 already and its been fine so far. I could do without the Firewall, i'm pretty safe with my machines to begin with
Out of the 4 machines, I actually have one where I purposely use it to test out the antivirus, so i go to a bunch of unsafe sites, download a bunch of dangerous files (basically, click all the things that a novice user would do) and its been pretty protective so far
With a free AV, I would normally run monthly scans always... but can't beat the price