Computer POST, then shutdown

[wingmann13]

So I am trying to fix my friends old computer....It will POST, and everything looks fine and then before it even starts up windows it shuts down. I'm not so sure about the heatsink on the CPU, it is old and maybe seated poorly. I don't have another heatsink to try at the moment but my question is could heat be the issue here? Would the computer shut itself down if it has a bad heatsink or is this a motherboard issue?

Asus P5ad2 mobo
Intel Pentium 4 3.4ghz
ati radion x800
1gb PNY 533 ddr2
Dynex 550watt PSU

 

[r-sky]

Enter bios... See if you can check the temps there before it shuts off.

 

[rdb]

Could be a heat thing. Should take only a second to see if the heatsink fan spins up. and a min. to relace the thermal compound.

 

[Onus]

Does it shut down after the same amount of time on, or even though it is quick, is it random?
If the shutdown is occurring consistently about 4-5 seconds after startup, check for a shorted power switch or wires. If it is more random and after a longer period of time, heat is more likely the problem.
Since you say the machine is old, another distinct possibility is that capacitors have failed on the mobo or in the PSU. If you see some that are visibly bulging or leaking, it's time to throw in the turban and replace whatever is bad.