Possibly could be it if you didn't fasten the HSF enough... though with the push pin system, it's rather hard to screw up there. Your speaker should give you a long warning beep until it automatically shut offs if your CPU was overheating. Make sure your speaker is connected properly first to be certain. One beep depending on the length and number (edited) from the bios could mean a lot of things but from what you have said, I get the feeling it sounds like your computer POSTs which means you should be able to see the start up screen and get into your bios. Is this the case? Does nothing show up on your monitor? Did you try reseating your electronic components like your RAM and Video card? I'm taking it you really did plug everything in.