You can also run a few free online antivirus scanners. I've successfully run them on client machines when the antivirus was acting up.
http://housecall.trendmicro.com/
(the older version of the scanner was beter)
http://www.pandasoftware.com/products/ActiveScan.htm
http://www3.ca.com/securityadvisor/virusinfo/scan.aspx
http://www.kaspersky.com/virusscanner
(does not automatically remove viruses it finds)
Kaspersky works the best, in my mind, but it's tricker to use since it doesn't automatically remove any viruses.
If you have always on DSL or cable, you should run these scans in Windows safe mode with networking.
http://housecall.trendmicro.com/
(the older version of the scanner was beter)
http://www.pandasoftware.com/products/ActiveScan.htm
http://www3.ca.com/securityadvisor/virusinfo/scan.aspx
http://www.kaspersky.com/virusscanner
(does not automatically remove viruses it finds)
Kaspersky works the best, in my mind, but it's tricker to use since it doesn't automatically remove any viruses.
If you have always on DSL or cable, you should run these scans in Windows safe mode with networking.