k more info is necessary.
(i) when u boot up, do the LED's on the keyboard flash
(ii) u say tht power goes to the mobo:how? does the LED on the mobo glow?
(iii) when u press the power button, do the fans start to run? does the led on ur optical drive (cd/dvd rom/writer) flash?
(iv) does any LED on the system panel glow or flash?
(v) if ur system is a p-4, hv u connected the 4 pin processor connector, if any?
(vi) if u had disconnected the leads for the cabinet panel switches, did u connect them properly?
(vii) do the LED's on the RJ-45 (LAN) port light up?
(viii) does boot up floppy seek occur?
Try resetting the CMOS to defaults. If you can't locate the jumper, take out the CMOS battery (coin shaped) and replace it after five minutes, n try booting up
If it is onboard AGP, hv u connected that lead properly?