VirusTotal is an excellent website which will quickly scan the file you upload with more than 40 antivirus engines from all the main security companies: AVG, Avira, F-Secure, McAfee, Sophos, Symantec and more. And in a few seconds you'll get a near instant, and very reliable report on what they've found.
If opening a browser window and heading to the VirusTotal site seems too much of a hassle, though, you can always use their uploader program instead. Once installed, simply right-click the suspect file, select Send To > VirusTotal and the program will upload it for you, then display the report.
Launch the VirusTotal Uploader separately and it'll also display a list of the processes running right now. If you're suspicious of a particular program then you can highlight it and have the file checked out in a couple of clicks.
You can also drag and drop files onto the Uploader and have them all checked out at once.
Or maybe you're concerned about a file you're about to download? Paste its address into the URL box, click "Get and upload", and Uploader will grab a copy of the file without ever saving it to your hard drive. Once the site has delivered its verdict you'll be able to decide whether you want the download, or not.