VirusTotal

VirusTotal is a web-based system that scans any uploaded file with more than 40 different antivirus products. You can use VirusTotal to check your own computer.  The simplest way is just to head to www.virustotal.com in your web browser and upload any file you're concerned about.  However, there's also an official downloadable Windows utility that can do this for you automatically, and which has some extra capabilities that aren't available via the web site.

To get VirusTotal Uploader, head to here and download it.  It's only 0.2 MB and installs pretty much instantly.  Now, when you run the program, you can upload a file from your PC, or from any URL.  You can also see a list of all the background programs and processes currently running on your machine, as shown in the screen shot below, and you can choose to upload any of those to VirusTotal for checking too. So it's a great way to ensure that your computer really is malware-free.