I replaced the standard AMD fan with a new Coolermaster fan. The comp worked fine for 2 days. Then everytime I start the comp, it turns off 2 secs after...the boot screen doesn't even come up. What could be the problem?
That could be a power supply. I had a similar problem, but my computer was turning off in 10 - 15 minutes. Try to get other supply and put it to your PC.
Check your fan connection. Make sure that it is plugged in all the way or none of the pins are bent or anything. The newer motherboards wont let you POST if it doesnt detect a CPU fan.
Definately suspect a problem with the installation of the new heat sink/fan.
Check it out.
Does the CPU fan spin? is it connected ok to the correct fan header on the mobo? is the heat sink correctly installed (this can be very hard to tell just by looking so take it off and re-install it). Did you use thermal paste and did your use it as instructed?