VirusTotal.com-- Free file scanning
 

If you download files from the internet, here's a little jewel of a website. VirusTotal is a FREE service that analyzes suspicious files and facilitates the quick detection of viruses, worms, trojans, and all kinds of malware detected by antivirus engines.
Go to VirusTotal - Free Online Virus and Malware Scan and provide the url or disk location of the file. It will scan the file with 20 different engines and quickly tell you if your file is infected. You can also download a small interface to your computer that connects to VirusTotal for quick scanning.
Let me know what you think.