System Shuts down

Hi,
I have a system which is AMD 800 GHz, 64 MB SDRAM PC100, a VIA motherboard with AGP Radeon and Lan card. My system shutsdown automatically. Even if i disconnect the HDD after i switch on my system and leave it for 3-4 mins, it shuts down automatically, i have changed my CPU, memory and tried but still the same problem. Does anyone have any idea how to slove this problem.
I also did the BIOS defaults, But it didnt work.

Regards.