Juice daddy is right, look in the mainbord manual and find the corrct pins for the power on button, then take a jumper ( the little black plastic with metal clip inside) and momentarily touch the 2 pins that the power on button goes. do not use any metal opjects. I lost a K6 mainboard doing this. I was holding the jumper with forcepts and slipped and shorted the wrong pins with the side of the forcepts.
..there was a slight spark and it did not ruin the board competely, just knocked out what ever option the pins were, which in the long run made it unusable. It has to be momentary also, if you leave it "on" the jumper it will start and then shut down, just like if you held in the button. Just make sure Timmy does not drip any sacrafical blood on the board.