Today Microsoft just released the Microsoft Windows Malicious Software Removal Tool. It checks your computer for malicious software, such as Blaster, Sasser, and Mydoom (for a full list, see KB 890830, and helps to remove it. Updated versions of the tool will be released on the second Tuesday of each month.
To have the newest versions automatically delivered and installed as soon as they are released, set Automatic Updates to Automatic. The version of this tool delivered by Windows Update runs on your computer once a month in the background. If it finds an infection, the tool will display a status report the next time you start your computer.
Note that this is not a substitute for anti-virus software, which you should also have installed on your computer.
When I ran the tool, it finished with this message: