This tool is not intended as an antivirus, it is intended as a cleanup utility for infected computers or as a tool you can download and do a double-check to confirm you are not infected (say your installed antivirus is unable to detect a certain virus/malware, then you can double check using Microsoft Security Scanner).
You can download it free from here (note the download only works for 10 days, then you have to re-download an updated version, this is to ensure the virus detection patterns are always fully up to date);
A good thing to also do is to run Microsoft Malicious Software Removal Tool from time to time – this is done automatically as part of Windows Update, however this is only the ‘fast/quick’ scan – by starting the MRT.EXE manually you can do a FULL scan. The Malicious Software Removal Tool is installed on all windows machines and updated/maintained via Windows Updates.
Btw; McAfee has a similar yet not so comprehensive utility called Stinger (also free download);