Trend Micro Rootkit Buster BETA is a portable, easy to use and extremely quick way to detect and disable some rootkits.
The program comes in the form of a single executable. This can check three main areas for infection - "Files or Master Boot Records", "Services" and "Kernel Code Patches" - and each of these can optionally be disabled, if you can think of a good reason to do so.
Generally, though, all you then have to do is click "Scan Now, and watch as Rootkit Buster scans the system areas you've specified, delivering its verdict and disabling any threats (if present) in little more than a second.
As its extreme speed suggests, this isn't the most thorough of security health checks, and Trend Micro Rootkit Buster is no substitute for a regularly updated, full-strength antivirus tool.
The program is easy and convenient to use, though, so it's worth keeping a copy around, just in case you're infected by something which your regular security package can't remove.