NOD32 isn't free. Avira AntiVir offers a free version and so does Avast.
I second Facebook in recommending
http://www.kaspersky.com/virusscanner . It's always my "second opinion" virus check. However, it doesn't offer removal. It merely shows what's on your system so you can remove them yourself.
If you want instant analysis and removal, go for TrendMicro Housecall:
http://housecall65.trendmicro.com/ (all browsers) or
NOD32 online scan (Internet Explorer only). After this is done, get an antivirus like
Avast Home or
Avira AntiVir. Stay away from AVG 8 (it causes lots of problems and slows down your computer).
Like Dynamics said, Avira has this annoying popup telling you to purchase their security suite after every update. You can disable it by following this
simple trick. Avast doesn't have annoying popups (it's the only feature-rich antivirus that is free out there) but has a lower detection rate than Avira.