Please list your statistics - ie proc, mobo, ram, etc
Test your processor in your friends PC. Try and test the major compenents of your machine: RAM, HD, re-seat and inspect the contacts on the video card, ram and and cables - even the Monitor...but I'm willing to bet it is the motherboard.
Even thought you get the lights, a bad capicitor or other defect might cause the problem and the fan gets power from a power tap or maybe even a lead from the power supply directly. And I don't think the four lights test for as muich as it does/shold with reguards to the enitre circuitry of the motherboard - I can't back this up though.
There have been motherboards with bad histories...ie made bad batch of low quality electronics that make it to market, check yours online to see if 'fits' w/ other complaints.. And I've had motherboards too close or touching something in the case which cause problems...
Hope this helps, I know that some of my suggestions aren't exactlly 'unobvious,' I was not intending to be pretentious. Hope you get it working!
There is no reason for any individual to have a computer in his home.
President, Digital Equipment, 1977