I was trying to give my old PC to a friend [the configuration follows], I needed to install a new PSU and a new fan [4cm in dia] replacing the old fan on North Bridge. Everything went well, and it was working. after a night my friend called about the CD-RW drive not working, and coincidently found out something weird about the case door and PSU which leads to no power for the drive. after a few days the whole system did not power up. I tested everything and the small orange LED on the Motherboard [which indicates power source doesn't turn on]. I changed the PSU, and nothing happened. I stripped the Motherboard off everything [CPU+RAM+on-board Graphics] and connected it to another case, and Voilą, it powered up, I tested the other PSU, and it was OK. [I didn't test it with a MultiMeter]. Connected the first case and PSU and it powered up again, checked it for 3 times, then connected everything, nothing happened, again no power up...
Now, it doesn't power up with the other PSU and case, I took the Motherboard off the case to reduce possible tension, nothing happened...
Tested the CPU on another MB ... [OK]
Tested another CPU on this MB ... [Fail]
Inspected MB for any visible damages ... [everything OK]
I'm going to test RAM modules, however, the Orange LED doesn't turn on, there is no power going to MB, so this is probably not the case.
MB: Gigabyte 8IG1000 Pro (Rev. 1.0x) with Intel 82865G North Bridge
CPU: Intel Pentium 4 2.4Ghz - 800 MHz FSB
RAM: 2x 512MB Kingmax DDR-400 working on Dual Channel
PSU: TCM 550 Watt
old PC no power up 3 days after PSU change 
Linear Mode
