Without knowing which OS you're running, I'll assume it's XP or W2K. What we need in order to be able to help you is the actual content of the "Stop Hex Code", something along the lines of...
STOP: 0x0000009C (0x00000004, 0x80545FF0, 0xB2000000, 0x00070F0F)
That information can be the essential clue to the underlying cause of your problem. Causes of your particular error (MACHINE_CHECK_EXCEPTION) are usually hardware related...overheating, overclocking, or an actual hardware defect (RAM, CPU, and MB are the most common.)