Does it actually post, after you left only mb, memory, cpu and hard drive?
Try removing the battery of the mb for half an hour or so, or look on the manufacturer's site on how to reset the bios to factory defaults.
Can you try the psu of your own computer on it?
If it won't post after changing psu, then it is either mb or cpu.
I agree you can probably get a low-end cpu/mb combo for less than $100.