Custom Mobo Power/Reset Switches

Hi everyone, I'm new to pc modding so please excuse my noobishness. But here is my story:

I picked up an old Dell computer for free. The copyright on the mother board says 1998 so that gives you some idea of how old it is. Its got everything it needs to run, minus the case, so I figure I'll just make a case and give it to my sister for her to use.

Everything goes well, case gets built and everything fits no problem. But I want to make custom power/reset buttons for it. The original Dell power/reset buttons are on a circuit board thats attached to the mobo via a small ribbon cable. (I'll try and get a picture of it tomorrow) Anyway, I take a switch from Radio Shack and I attach it to the two pins on the mobo that I believe to be for the power switch. Then I hook up a power cable and throw the switch, computer boots fine for 5 seconds, then shuts off.... try it again... boots for about 2 seconds... shuts off... I try the original Dell configuration and it boots fine. So any idea why it does that? Any help at all is appreciated. Let me know if you need more info on anything.