It sounds like it's stuck in a loop - it will try and start up but something makes it reset itself. Try taking it apart and unplugging everything except the power supply, graphics card (leave the monitor plugged in as well), memory and processor. Then start it up again and see if it starts as normal with 1 beep. It should get as far as saying non system disk or disk error when it can't find Windows. If it does this plug everything back in until you find the part that's causing it to reset. If it still resets when you just have the power supply, graphics card, memory and processor plugged in then it could be a fault with one of these components so try different ones out.