Well, there could be quite a few things...
But, here's the top few that might help.
1) Download the Mac OS 10.5.8 Combo-Updater, and run that...
2) Check Virtual Box, and make sure that you have the latest version installed, along with the virtual box tools in your windows session / boot camp.
Let us know what happens.... And try checking your memory as suggested with the other answers...
Try reseting your RAM, and Hard drive. Disconnect all USB & Firewire devices, and if possible try running Tech Tool Pro, in extended hardware tests...
Something is obviously causing a problem... But there doesn't seem to be a obvious reason for it.
What antivirus packages do you have loaded? And do you have Virtual PC installed? Is there any software packages that you have installed or updated recently?
It just works! Except when it doesn't. – hyperslug – 2009-10-01T04:37:22.030