Weird system shutdown on startup.. Help!!

I built a computer using all components from my older computer which worked perfectly before, just sold them to a friend and he asked me to install it into his case. Anyway i finished and pressed the power the button. The computer starts, i get the post screen message then 2 beeps and it automatically shuts down.
The weird thing is, the first time i press it, it goes to the select windows screen, then 2 beeps and automatic shutdown. Then when i press the power button again, it reaches the post message and 2 beeps and shuts down, the more i press the button the least time it is turned on. Eventually the instant i press the power button, it instantly shuts off.
But say i wait about 5 minutes and turn it on, it all repeats. Reaches windows select and shuts down, so on so on untill again i have to wait 5 minutes.

The only different parts that are in the computer from what i had before is the standard q6600 cpu fan (i had watercooling), my friend bought a 750w psu (i had 700w), and the ram is brand new.. I think its not the ram problem because like i said at first start after taking a break it passes the ram check, then 2 beeps and so on. The weird part is that everytime i turn it on, the time between shutdowns gets shorter?? Please i really need help, need to solve this.

Yes either the CPU fan isn't turning, the heatsink isn't mounted right, or the PSU is bad and overheating. Or it could be the some part on the MB overheating, but that's less likely.

Did you have the processor overclocked?

What make is your motherboard? I just checked a few places and it seems 2 beeps is commonly a post error or a memory error but beep codes vary by manufacturer so check with your specific manufacturer