What is Windows Malicious Software Removal Tool?

Every time you run windows update, you will see “Installing Windows Malicious Software Removal Tool - ...”  you wonder what is this, how I can use this app. or do I still need antivirus software?

Microsoft released this free distributed antivirus tool that scans the computer for specific widespread malware and eliminates the infection, on the 13th of January 2005,  It is automatically distributed to Windows based computers on second Tuesday of every month via Windows Update service and runs once automatically in the background and reports the malicious software report anonymously to Microsoft , can be disabled

Alternatively, you can manually download this tool from Microsoft Download Center. It records its results in a log file located at %windir%\debug\mrt.log. You can run "mrt.exe" at any time from command prompt

In a June 2006 Microsoft report, the company claimed that the tool had removed 16 million instances of malicious software from 5.7 million of 270 million total unique Windows computers since its release, on average, the tool removes malicious software from 1 in every 311 computers on which it runs. As of 19 May 2009 Microsoft claims that the software has removed password stealer threats from 859,842 machines