Well now, time to diagnose!
My initial guess is that the problem lies within either the motherboard or the hard drive. So my suggestion is to deduce down to which one it is.
How?
To figure out if the motherboard is the problem, I would unplug the hard drive's data cable from the motherboard, and then power up the computer. If your motherboard is okay, you should be able to get to the BIOS screen without any problem, as the hard drive of course is just a boot device to get from the BIOS to Windows. Without it connected, you just won't be able to load windows. Try pressing buttons like F2 or Ctrl-D or all of the F1-12 buttons when you first turn on the computer, to see if you can get to the BiOS settings.
If you don't get to the BIOS screen, then voila! You've figured out it's the motherboard! Right? Well....... not really. At this point you've only figured out the hard drive is not the problem. It could be the RAM or the CPU or the motherboard -- and possibly the videocard, but you can safely unplug that and still be able to get to the BIOS screen. EDIT: Well, you
can get to the screen, but you wouldn't be able to see it.
If you CAN get to the BIOS screen, then it's almost definitely the hard drive. Now you should have the data cable unplugged, but the hard drive's power cable should still be in. When you power the computer, do you hear it revving up? If you can't hear it, can you feel it moving? (Just put your hand on the outer case of the hard drive.) If you can't hear or feel it powering up or spinning around, then try switching power cables. If it STILL doesn't power up, then you can be pretty sure it's the hard drive.
OoooooOoooh.... this flowchart looks helpful. For diagnosing RAM and CPU this looks like a lot of help:
http://www.fonerbooks.com/cpu_ram.htm