Assuming the PC all worked before the cooler change, then it won't be the following..1. It won't be GPU
2. It won't be memory - unless you re-seated it in the process, in which case, why?
3. It SHOULDN'T be the PSU unless the cooler requires a molex or similar plug -
HOWEVER - if you changed cooler, check that you've not unplugged the 4/8PIN motherboard power connecter by accident during the process
I get it to startup and it runs for about 5 seconds and then the PSU shuts off. It then starts up again and continues to cylce but never boots. I never hear it get to Post.
Did it "beep" before you installed the cooler?
Which cooler did you install?
Did you clean off the original TIM? If so, what with? You may have got TIM on the circuitry or your cleaner that you used that may have shorted the board.
Check the 4/8pin power motherboard power connector hasn't come loose or came out.
Check the cooler mount isn't making contact with the board (metal clips, screws etc) - if you're using a mounting plate make sure the washers are correctly in place or this will short the board.
~Bex