The only reason I didnt think it was the powerbrick or voltage regulator was because in those cases most users will not see any activity at all, or it will only boot on battery power. Since, he has no battery it makes this a bit more difficult.
Now that his MOBO and voltage reg. has been replaced, and the issue is still there it has to be some other component. Since the problem first started when he was switching out Processors it might make the most sense to test that first. Reseat and test it, then swap it out if possible. If none of this works move on to the next component like RAM, HD, etc. Since you should now have a new/working MOBO and voltage reg I we can assume this parts are working and through the process of elimination find out what is malfunctioning.
Truthfully, from your description of the issues it did sound like a MOBO issue. Since it was replaced it could be anything and must be tested for. As a general guideline if u can get any activity at all when you try to start it, its most likely not a Power Supply issue. If you can get up to the windows boot screen its most likely bad memory, HD or other component. If it seems like its booting but displays nothing, its probly a video card issue. If the errors dont fall into anything above, and you are seeing irregular behavior from your PC its probably the CPU.
Either way you will have to indivually test each component and record your results so we may narrow down possible failures.
PS; Are you sure the replacement MOBO is functional?