ube,
Do you know for a fact that the PS is good?
If it is, then the only thing I would suggest is to verify your Power connections to all peripherals. The last time this happened to me, I bent one of the four prongs on the power conenction for my floppy. This was causing the entire system not to get any power.
If every connection is correct, then unplug everything except for the video card, CPU, RAM and monitor. Start the system and plug the pieces back in one by one until you figure out which one is causing problems.
Good luck!