Windows Malicious Software Removal Tool
Normally is installed via Windows Update and runs automatically in a silent mode and only reports if infection was found.
It is not a replacement for Antivirus software as it only finds active malware and does not prevent new infections.
To run manually: Start > Run > mrt.exe
You can choose Full Mode if you wish (by default runs in quick mode)
Creates a log file: C:\Windows\Debug\mrt.log
Windows XP, Vista, 7