Virus Total, another tool for us to use,......

So you have a file that seems suspicious and your Anti-Virus program finds nothing wrong with it?

You can now send it to the folks at Virus Total and they'll scan it with multiple virus scanners and give you the results.

Here is their explanation of the free service:

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About VirusTotal

VirusTotal is a service that analyzes suspicious files and facilitates the quick detection of viruses, worms, trojans, and all kinds of malware detected by antivirus engines.

Specs:

• Free, independent service
• Use of multiple antivirus engines
• Real-time automatic updates of virus signatures
• Detailed results from each antivirus engine
• Real time global statistics

Great site, I've been using it for a few years now. One caveat though is because it scans with so many scanners false positives are very common. so you need to take the results with a grain of salt. Another handy piece of advice if they show you cached results you might want to resubmit the file to make sure they use the latest definitions from each vendor to scan.