I am trying to help a friend of mine with his computer. His power supply fan is giving trouble, he bought a new one but the design of the current supply he has is way too small to fit the new fan. Any suggestions as to what to do?

why not buy a new power supply with higher capacity..instead of buying a power supply fan

Well easiest way to test this, is to take one power supply, and plug it into another computers motherboard, just to test it and see if it turns on.

Most likely, this sounds like a motherboard issue, or a ram issue.

If your motherboard is burned out, it will shut off and tell your power supply to do so as well ( in most cases ).

The motherboard can detect usually things such as defective RAM, and it will automatically shut itself off to prevent any further damage.

#1 It is not your hard drive, cd rom, disk drive or any of that.

Its either
A Your mother board,
B all 3 power supplies are bad because they burned out because the motherboard isnt properly installed or they got a surge. or
C Your ram is burnt out and the motherboard knows it, so its auto shutting off.

You can eliminate B by testing the supply in another pc, you dont even have to install it, just plug it into that pcs board and the wall and turn it on then off once you know.

You can eliminate C only if your pc actually works and you run a memory test ( also you could buy some new ram or if you have 2 sticks of ram, try removing one, and then the other to see if that works.

And to eliminate A, you would just pretty much have to buy a new motherboard to test the theory out.

The easiest way to fix this problem i would think is to just buy a new power supply. You could get a more powerful power supply and also buy a never more powerful fan to cool it down as well. You may have to find a new location in the CPU for the new fan.