It can be anything. Hardware failure. Windows Vista becoming unstable.
The best thing to do is immediately backup your important data.
If it is a software issue, then it would be difficult to determine what the cause could be. Did you recently install any software? Or have you shut off your laptop without allowing Windows to properly turn itself off?
A clean install of Windows Vista is probably the best option to take if you are not going to bring your laptop to a repair center. If after a clean install you still have BSOD (blue screen of death), then that would point to a hardware issue. At that point you will need to bring it into a repair center.