All of these are well-respected & widely-used anti-virus software & free:
Only use 1:
AVG anti-virus
http://free.grisoft.com/doc/downloads-products/us/frt/0?prd=aff
PC Tools AntiVirus
http://www.pctools.com/free-antivirus/
Avira AntiVir
http://www.free-av.com/
Keep it updated, as new viruses hit the net daily
There's also these free online scans:
http://global.ahnlab.com/global/products/myspyzero.html
http://onlinescan.avast.com/
http://www.bitdefender.com/scan8/ie.html
http://www.ewido.net/en/onlinescan/
http://usa.kaspersky.com/products_services/free-virus-scanner.php
http://www.pandasoftware.com/activescan/com/activescan_principal.htm
http://www.trendmicro.com/en/home/us/personal.htm
http://onecare.live.com/site/en-us/center/howsafe.htm
http://www.ca.com/us/securityadvisor/virusinfo/scan.aspx
.