Microsoft Windows Malicious Software Removal Tool is a freely distributed virus removal tool developed by Microsoft for the Microsoft Windows operating system.
First released on January 13, 2005, it is an on-demand anti-virus tool ("on-demand" means it lacks real-time protection) that scans the computer for specific widespread malware and tries to eliminate the infection.
It is automatically distributed to Microsoft Windows computers via the Windows Update service but can also be separately downloaded.