I have had this problem frequently on my imac which is an upgrade from Tiger. Now it's happening on my newer macbook which came with Leopard. Often, when it wakes having been asleep for some time, things stop responding. The HDD sounds like it is furiously writing, the pinwheel appears, programs are either slow or don't respond (I can't even run Activity Monitor) and then it restarts on it's own. I've tried repairing permissions and all of the options in disk utility. What might it be?