Re: What am I doing wrong?
This could be one of a few reasons, the most common is the on/off power switch at the front of the Computer hasn't been connected correctly to the motherboard.
Re-check that all the cable/wires from the power supply and the switches/lights at the front are all correctly connected.
Oh and if the Computer powers up e.g you can hear the hard drives fire up, then check that non of the IDE or floppy cables are in the wrong way around, on some motherboard this prevents it from post booting.
AMD Athlon 64 X2 Dual Core 4200+
DFI LanParty UT RDX200CF-DR Crossfire Motherboard
OCZ 4GB (4 x 1GB) PC3200 Dual Channel Platinum Series CAS2
ATI Radeon X1950Pro - Crossfire Edition - 512MB GDDR3 x2 cards installed.