First and formost, try a different power supply. I've seen this happen on PC's many times where the system powers on, the leds light up, but no video and no "post beeps"... Usually changing the power supply will take care of it. Although, it could be signs of something even worse....
Also try to reseat your AGP card, as most AGP slots require an extra push to get the card seated all the way down (not with excessive force though). It may look like it is inserted, but may not be seated completely down. That will also cause no video, but shouldn't make the cpu fan not turn.
