VirusTotal is a site where you can upload a file smaller than 10 MB and it will be scanned by a large number of antivirus software (the current number is 31), including: Kaspersky, BitDefender, F-Secure, Panda. The file will not be scanned instantly, but you'll have to wait a short time (usually around one minute), depending on site's load. You'll get a report like:
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If you see conflicting responses, look at the most trustworthy engines (some tests) and at the number of engines that report a virus. In the situation depicted in the screenshot, I can safely assume that the file is clean.
The service is available by email too: send a mail to scan@virustotal.com with the subject SCAN. If you use Gmail, you'll have to rename executable files (for example, from setup.exe to setup.ex1) to be able to send them.
A similar service is Jotti's malware scan, that has a bigger limit for the file size: 15 MB, but uses less antivirus engines.
{ Thank you, Google! }