Microsoft Security Scanner is free virus removal software from Microsoft that scans your PC from virus and malware, and removes them. It provides an on-demand scanning option that you can use to anytime scan your PC. It is not a replacement for a free antivirus, and does not provide any real-time protection. It provides a PC safety scan.
It is a helpful virus scanner to get a second advice if you think your PC might have a virus that your anti-virus did not report. You can download this virus scanner for free, and it will thoroughly scan your PC for virus, malware, and spyware.
[advt]The download of Microsoft Security Scanner is quite heavy: around 70 MB. Of course, that is because it has to download all the latest virus definitions as well. Also, the downloaded version works for 10 days only, and you need to download a fresh version after that. This is to ensure that you always use it with latest anti-virus definitions.
If you are using Microsoft Security Essentials, then this virus scanner might not be of much use, as it might be using same virus definition files that Microsoft Security Essentials might already have.