F-Secure provides an excellent free online anti-virus and malware scanner

If you suspect that your PC is infected with some sort of virus or malware try the Free F-Secure online anti-virus scanner. It works only with Internet Explorer but is an excellent scanner that pretty much caught everything in my PC that other scanners missed.

Here is the link:
http://support.f-secure.com/enu/home/ols.shtml

I would suggest running this can along with atleast 2 more anti-virus/malware scanners such as AVG (Free Edition is fine) and MalwareBytes. Leaving the scanner to run overnight is a good idea as the process is time consuming.

AVG Free Anti-Virus
http://free.avg.com/download-avg-anti-virus-free-edition

MalwareBytes
http://www.malwarebytes.org/

Panda ActiveScan
http://www.pandasecurity.com/homeusers/solutions/activescan/