Here is a list of the online virus scanners free to use. They come in handy when your installed antivirus isn't able to detect an infection, and there isn't a specific cleaning tool available to clean off the virus. This list is in no particular order. Some scanners will require you to use Internet Explorer and install an ActiveX control, and others just upload a single file to scan.
Trend Micro Housecall
eTrust Antivirus Web Scanner
BitDefender Online Scan
F-Secure Online Scanner
Kaspersky Online Virus Scanner
VirusTotal File Scanner