Home to some of the most useful software on the planet - and it's free, no less - Sysinternals Freeware is for the System Administrator that has almost everything. Their newest release is a rootkit detector for Windows. It will find hidden registry keys and files that may provide the hiding locale for hidden bugs on your system. A rootkit is installed software that hides from virus scanners and the like, and gives administrative control (root in Unix terms) to the programs it's running. The 'kit' means that it's a utility with multiple skills, and is installed all at once by hackers to attempt to take over your machine and hide itself from detection. My favorite software they distribute, however, is Process Explorer - it lets you get into the nitty gritty's of the processes running on your machine, and can help you debug some of the nastiest problems if you know what to look for. I highly recommend this site.