I would offer the following as a starter. Freezing of the mouse may indicate a faulty mouse... I had this happen recently on a less than two year old computer. First try another mouse and see what happens.
As to the hard shut down that follows I can offer no solution. Hopefully the mouse may be the whole problem. Cheap and easy to fix.
I am using IE6 with Win XP on a Dell desktop PC. Recently, the mouse pointer freezes when I push the down scroll button in the browser window. The pointer just sits there and is completely non-responsive to mouse movements. Then a few seconds later, the whole PC resets itself (not a controlled Win XP shutdown, but an immediate hard reset as if I had switched off the power for a second).
I've had this same PC for about 2 years, and I never had this problem until recently. Any advice would be greatly appreciated!