Computer turns on for about a second, then off.
Mu computer (ASRock K7S41GX, AMD Athlon XP 2700+, 2x512MB Ram, PNY GeForce 6200) turns on, fans and hard drives spin, and about a second and a half later... it turns off. I then have to unplug the power cable, plug it back in and try again. Need help ASAP.
Power supply is the first thing I would check. They do go bad eventually, especially on a system that old. If you have another PSU that you can use (or borrow), try hooking that up and see if it solves the problem.
Another possibility is that one of the power cables came loose, and that's causing the motherboard to get confused about how to distribute the power and lock up. So I would check all the cables, make sure everything's seated properly in the slots, etc.
A couple of other possibilities: The motherboard is shorting to the case -- check the screws and standoffs holding the motherboard in place to make sure nothing's come loose, nothing's bent, etc. Or, something's going wrong with the video card or the motherboard itself (e.g. a bad slot not providing any power). Though I would check the other possibilities first.