I am running Ubuntu 9.04

This a recent problem, and so far has only happened twice.

I am working and the machine freezes. Keyboard and mouse buttons do not work. What is strange though is that the mouse pointer works but changes to a hand, or more like a fist.

The only solution I have found is to shut down and start again... a bit frustrating when I am in the middle of something.

Any suggestions gladly received.