The easiest, most bonehead proof method is merely to plug your PC and pump into the same power strip. Turning on the strip starts the pump, but you must still hit the power switch to fire up the PC.
The one above about the green wire to the mobo is not correct. This is what starts the PC, but depending on your BIOS setup you may find that holding the switch for four seconds turns off the PC. If not, pressing the switch once the PC is running often puts it into standby. In any event, running a flowmeter in series or parallel with the case switch is not what you want.
Relays are very easy to work with and do not require a soldering iron. You will, however, need to strip some wires. The ready-made kits are over priced, IMHO. You can pick up a relay from Radio Shack for <$5. Solid state relays are more reliable and can be had for <$20. The kits use the same sort as the cheap RS ones, yet cost you over $20 with S/H.
For the extra measure of security you do need some more electrical smarts. I'm an ME running a mechanical engineering department, but fortunately I've got a counterpart running the electrical engineering group. I wrote up a spec and he designed a system with a couple of relays, a pressure switch, and a timer circuit. One relay starts the pump and a timer when the system starts. If the pressure switch doesn't pull in prior to the expiration of the timer, the second relay kills power to the PSU. Brutish, but effective.
No need to really start the pump first as the block is capable of handling the heat of the CPU for a few seconds quite easily.
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