He needed a new fan installed cuz the one he had installed rattled. It was still operational but he got tired of the grating noise. I tried using sewing machine oil (reccomended) and it didn't quiet it at all. So then he bought a new fan. I neglected to check if the new fan were compatible with the motherboard until I disconnected the motherboard from everything and removed the motherboard from the case. The motherboard uses a 478 mounting slot and the fan that he bought, a thermal take 2 uses a 775 slot. After finding out that the new fan was not compatible with the motherboard I proceded to put everything back the way I found it. I wiped off the old thermal compound off the processor, applied new thermal compound and put the old fan and heatsink back into place. I then turned on the computer only to have it start up for 2 seconds tops and then shut off. The fans are spinning and everything seems to be in place, this is my predicament.

I need to know how to narrow down my options to
1) I'm dumb and connected something wrong or
2) I'm careless and somehow messed up the board and/or the processor