Look for an autorun.inf in the root of C:\
In all my years of using various versions of DOS, Windows, and Linux, I've never even heard of this quirk until it happened to me. I have Windows XP Professional installed -- a recent re-installation after re-building this computer -- and when I click on MyComputer I get the usual listing of drives, etc. However, when I click on the C: drive, I get a listing of my browser's bookmarks toolbar rather than a listing of the sub-directories on my primary hard drive. Clicking on any of the other drives shows the directories and files, as I would expect.
Any clues what might be causing this strange behavior on my C: drive?