Microsoft Security Essentials:
I have been very happy with it, but on my current machine, the scans were taking forever and ever.
Avira:
I used to love it, but I've had too many problems with it on too many machines.
ESET Smart Security 4 (x64)
After MSE slowed down, and then Avira started screwing up, I downloaded a trial version of Smart Security. It took a few days to get used to, and I had to disable its firewall, as it made my Internet connection very slow. After a little break-in though, it's not so bad.
Your thoughts?