Hmmm...
A coworker is suffering with a computer (Dell laptop w/XP) that seems unresponsive at times. The big clue was that sometimes the mouse cursor was not able to move onto the task bar. I googled and came up with some info, but that was a while ago and I can't remember the details.
I don't know if he ever fixed it- I'll ask.

edit:
Ah- the google search was still in the FF wonder-bar cache:
"windows XP task bar freezing"
It's a long shot that this has anything to do with your problem, though. Googling the symptoms might be worth a try.