What the MSRT Is:
The Microsoft Malicious Software Removal Tool (MSRT) is delivered monthly via Windows Updates as an "Important" update. Depending on whether you have Automatic Updates enabled and how it is configured, the MSRT download may occur automatically, or you may be prompted first. Note that malware typically disables the Windows Automatic Updates feature so it's important to periodically check to ensure this feature is working. (See: How to Enable Windows Automatic Updates).
What the MSRT Is Not:
MSRT is not a barometer of malware prevalence. Data collected from the MSRT is published in the Microsoft Security Intelligence Report. While the Microsoft reports very specifically identify the source of their findings, the data is still sometimes misquoted or misunderstood by the media.
Understanding Malware Prevalence:
These limitations do not mean the MSRT isn't providing a valuable function. The Microsoft Malicious Software Removal Tool is specifically designed to remove particularly prevalent malware families that have been identified as problematic by various global security organizations. And because it is delivered via automatic updates (or via the Windows Update center), it is accessible to the majority of Windows users.
In summary, the Microsoft Malicious Software Removal Tool is a very accessible (and free) adjunct to existing antivirus protection, but it is not a replacement for antivirus and its detection data is not indicative of overall malware infection rates or of overall malware prevalance.