You Might Try...
If you're of a mind to fiddle around with it you might try disconnecting all drives (HD, Floppy & CD) except your boot drive. Unhook any extra fans (not CPU!). If it boots immediately then I would agree that your power supply is not doing what it should. Having the extra load off the system might leave enough juice to make things work. Obviously you'll want the other stuff back, but fortunately a power supply is very easy to replace. (Unless you've got an old emachine, in which case you'll spend $115 for a 150W proprietary form-factor replacement).
Prof
(Even if the stuff I suggested doesn't let you boot normally it could still be the power supply, unhooking the extra stuff just gives you a chance if the PS is right on the borderline between making it and not quite supplying enough juice. You might also want to check your temps).