Im not sure, but I think it could be your power supply. It sounds odd I know. But this happened to a Dell of mine years ago. It would boot and then immidiately blue screen. Its something to do with the motherboard not recieving sufficient power.
Have you installed anything new at all recently, and how old is your pc?
Try taking everything un necessary out e.g. usb devices, cd drives, pci cards that are un needed. So it is at just enough to run and then see what happens.