magnethead

While I will already say this is unadvised (otherwise Dai will chew me a new one), you could splice the -12 VDC line and hope there's enough amperage for both the fan and mobo to run.

Your other choice would be to hit up radioshack or walmart for a 24 VDC wall adapter, molex to bare wire power adapter, and a simple single pole relay.

You'd leave the transformer plugged in full time, and use the relay to power up the fan when it senses a molex power feed (the computer being on)

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For proper support: what are you running? video card, cpu, m/board, ram, power supply, brand, wattage, any error message?
PCI-E requires 26 amps at 12 volts. That's a 650W PSU.
Power Supply Info and Selection . Info on thermal compounds & application . TEST PSU USING MULTI-METER