Your anti-virus vendor might be a few days (or more) behind on particular viruses. For this reason, it’s good to scan your computer with an online virus scanner. Symantec, Panda Software, Trend Micro, Kaspersky, and F-Secure are among those that have good online scanners that just might detect a virus that your installed anti-virus program missed.
Links for online virus scanners:
BitDefender (bonus – free removal):
http://www.pandasoftware.com (look for the ActiveScan link on the home page)