Many things to check; some simple and some not so simple. You said "PW Switch". Is that the only one of it's type? Some may be just PWR LED, etc. May be under a different "name". Also, try just "shorting" the two pins with a screwdriver blade for a second (bypassing the switch), to see if that works. In addition, make sure the CPU is inserted correctly along with the HSF being plugged into the MB in the correct spot. Next, make sure the power selection on the PS is set for 115 volt vs 230. Also check the positon of the on/off switch on the PS (sorry, had to ask). What MB, Power Supply and CPU is this? Are any of the fans or drives powering up at all? Take an extra second or two to go back over the connections with the board. Make sure only the basic items are plugged in; only the CPU w/HSF, the video card, RAM and floppy are plugged in to start.