From Microsoft PSS Security Team:

[What makes a 'software removal tool' malicious?]Announcement of Upcoming Release of Malicious Software Removal Tools
Starting from January 11th, 2005, Microsoft will provide Windows customers with Malicious Software Removal Tools. New versions of these tools will be available monthly (second Tuesday of every month on the same schedule that Microsoft already delivers other security updates) or more frequently if necessary.
These removal tools are an extension of virus or worm specific removal tools that Microsoft released in 2004. While tools released in 2004 have been specific to a single virus (and some of its variants), the new removal tools provide more convenience for customers by rolling up all viruses and variants targeted into a common removal tool.
Microsoft will provide new versions of this tool updated to remove malicious software that is found to be prevalent for that month. The first version of the tool available in January will be able to remove Blaster, Sasser, MyDoom, DoomJuice, Zindos, Berweb (also known as Download.Ject), Gailbot and Nachi viruses / worms.
These removal tools will be made available to customers through the following delivery vehicles:
- As a download through the Microsoft Download Center
- As a critical update through Windows Update and through [Automatic] Update for those customers who have [Automatic] Update turned on
- As an ActiveX control also available at www.microsoft.com/malwareremove