By the way you talk, I'm guessing you know your way around a computer, but I'll type this anyways.
First, strip the whole computer. Put the CPU, and motherboard hooked up to the monitor and keyboard, see if you can get into bios. If yes, then add one component at a time until it won't boot.
If no, try resetting the bios with jumper, or removing the battery.
If all else fails, I found out that my local "charge your ass of to fix your computer" repair shop will test EVERY component in that I bring them for a flat charge of $25 (NOT per component, I brought in about 10 things to test, and it was $25)
Now, IMO, $25 for someone to say "Your PSU is emitting irregular voltages, you need to replace it" is WELL worth it... especially since, in that instance, I had already ordered a new cpu and motherboard thinking I fried the mobo....