avira is quite good, freeware and doesnt slow ur system too much down.

Using McAfee ... great antivirus software. Recommended

I use AVG and haven't had any issues

In my opinion Kaspersky is the best. I have been using for more than couple of years and I never had any issues like re-installing OS cos of a virus/malware etc. (KIS 2011 a good option)

I use MSE Microsoft Security Essentials. Free for windows users, it has better detection rates for me than avg and has low system resource usage.

avira is good

avira premium security suite
avg is also good

Bit Defender is the best total security in the world because it detect viruses that other anti virus don't detect

Kis 2011 here

Kaspersky is good.

I like avg its free