No one should find what the problem is given this much detail. But let's start with the 2 usuals.
1. Malware. Take Grif's advice from http://forums.cnet.com/5208-6142_102-0.html?messageID=3105025
2. Heat. Are you doing your yearly blast of canned air to the vents?
Can anyone help me...this computer keeps randomly crashing and I have no idea why! Is anyone else experiencing this? If so how did you resolve the issue? Please help!!