Hard drive is probably fine. If it won't POST, then it is failing before it even tries to talk to the hard drive, so that's not the likely failure point.
First thing to try is to pull the power cord from the back of the computer. Wait a good 60 seconds. Then put the power cable back into the computer, and try to boot it.
If this fails, then I would try the following:
1. Reseat every cable and connector on the motherboard. This means all the power connectors, the drive cables, and the small wires that connect the mainboard to the power switch and sound ports and usb ports.
2. Still no workie? Reseat the RAM. Unplug and reinsert all peripheral cards.
3. Still not working? If you have any peripheral cards other than video, remove them and try booting. If you have built-in video that you're not using because you have a good video card, remove the video card and use the built-in video port and see if it boots.
4. No dice? Buy a new power supply from someplace with a good return policy. Disconnect the old power supply and connect the new one. If the machine boots, then remove the old power supply from the chassis and install the new one. If the machine doesn't boot, then return the power supply and get your money back.
5. No joy? If you're feeling ambitious, reseat the CPU, which means uninstalling and reinstalling the CPU cooler, so have some thermal paste handy.
6. Last step if its still not booting: if you happen to be blessed with a lot of compatible spare parts or a nearly identical spare machine, its time to start swapping parts, one at a time, until you locate the bad part.
Otherwise, it's probably a bad motherboard.
Good luck, and let us know what happens.
-Llama
