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?