Question Computer turning on then off after one second.

[badnatey]

i7-2600 at 3.4Ghz/3.8Ghz turbo
2x4 (8GB DDR3 total)
500GB Seagate SSD
R7 265 2GB video card
Antec 380w PSU

Not sure the make of the motherboard, but the PSU had a 24 pin connector. We tried a different PSU but it only had a 20 pin connector, so I'm assuming that it wouldn't work? It's a real old, real cheap PSU from a pre-made HP.
The fan comes on for a split second when the power is pressed but stops obviously.
I'm not super computer savvy, but I can take them apart and reassemble them. I just have no idea about anything else.
This is my siblings computer. Mine is more high end, so I'm not use to seeing parts that are more low-end.
Any help or thoughts would be greatly appreciated.

Later steps;

I did try to start my computer with their PSU and it worked. So it's not the PSU. Also replaced the battery on the MOBO. Reseat ram, checked all connections.. nothing.

So MOBO or CPU .. which would be the issue? Or am I missing something else? Anyone have any thoughts?

 

[Lutfij]

You will need to post back with the make and model of the motherboard. Then if the board has a 24 pin connector, you're going to need 24 pins from the PSU. How old is the 380 W PSU and what is the exact model of the unit? The other matter is, doesn't the system have a discrete GPU?

 

[badnatey]

You will need to post back with the make and model of the motherboard. Then if the board has a 24 pin connector, you're going to need 24 pins from the PSU. How old is the 380 W PSU and what is the exact model of the unit? The other matter is, doesn't the system have a discrete GPU?

What's a discrete GPU?

And as I stated, I hooked up the 380W PSU to a different PC and it powered it on fine. The new PC I hooked it up to is a much higher quality build as well. So I don't think it's the PSU anymore.