The BSOD is usally caused by routine hardware failure, but not always... The most common is memory, but it can be as simple as an unused floppy drive, any device in a PCI slot that has decided to fail or has had a driver failure. It can be a power supply failure or a worn CPU cooler fan... or any other powered device that requires driver software.
Now that you have ruled out memory, rule out CMOS batter, then each of the other powered conmponents.
You can begin by going to Start->Control Panel->System->Hardware->Device Manager and look for any yellow or red flags. Replace by upgrading any item so marked.