[BiPolar_Nut]

I mostly agree w/ the above post, with one alternative to suggest.

It could just be a worn out power switch.

To test the power switch theory, you'd do the "shorting it" thing which you may or may not be comfortable with. Basically, most of the front panel stuff (power and reset buttons, HD and power LED's, PC speaker) all plug in in the same general area on the mainboard. You;d need to determine which wire pair is for the power switch and short those 2 pins. Most mainboards have separate plugs each for the speaker, power switch, reset switch, hard drive LED, and power LED. You'd simply remove the 2-wire connection labeled "power sw" and use a knife or something to short those 2 pins. Other brands (like most dells) have one big fat connector with all those connections on one big plug. In that case it'd prolly be easier to get a couple pins (like from a new dress shirt), and pierce each of the 2 power switch wires and touch the pins together (this won't break the original functionality of the switch).

If shorting the pins works reliably....then you simply have a worn out power switch. (which personally I'd either rig w/ any kind of momentary SPST switch I had laying around or replace just the computer case, which could range from $15 to $250 depending on the design and features))