http://housecall.trendmicro.com/ This is a good online program
Another exceptional cleaning tool (scanning only, it doesn't do realtime for free) is MALEWAREBYTES
http://download.cnet.com/Malwarebytes-Anti-Malware/3000-8022_4-10804572.html?tag=mncol
There are several listed here:
http://download.cnet.com/windows/ant...g=ltcol;narrow
Threatfire is a good behavior based program that I recommend AFTER you clean up your system. It isn't that good at cleaning an already infected system.
AVIRA has the best reputation.
I personally like AVAST! but its effectiveness has recently come into question compared to other free progra.
I installed AVG and Threatfire together on another PC and they work well together (behavioral plus standard script scanning). If you do that remember to put threatfire in AVGs "exception" feature for realtime scanning.
Also if you change your dns server to OpenDNS (free) it give you another layer of protection because they have great antiviral features. If you (or a spamware program) tries to point your browser to an infected site, it simply won't go there:
http://www.opendns.com/ (you don't have to use any of the blocking features)