I've had this problem a few times before, each time a different problem but same effect:
1) My computer booted but monitor standby...I later found while opening my computer case, a large dust bunny inhabiting the fan/heatsink area of my graphics card hence frying it. I had the card replaced though I haven't bothered cleaning my PC since
2) After fitting a new motherbord, I had the same problem, to find I had a few screws loose causing a short on the board or something
Computer turned on but there was some weird pulsating effect, the fan lights blinking and fans going wild...
3) After fitting in some old ram I had lying around, I had the same problem again...I removed the faulty RAM module throough trial and error and it all worked well