Gotcha. Even my three guesses were off. Duh.
It's this problem:
Tooltips for some taskbar icons are hidden behind the taskbar on a Windows
XP-based computer:
http://support.microsoft.com/?kbid=912650
Not that the KB's solution is that great. In Windows 98, if I recall
correctly, I dragged the Toolbar to the top of the screen, making it
temporarily a Mac, and then drug it back. (Acknowledging that some solutions
don't work for all computers, here.) I think on XP I Un-checked "Keep the
taskbar on top of other windows", Applied it, and then re-checked the box,
and Applied.