Sorry about the previous post, I was being a bit dim.

This is what you want.

Two 12v relays.

You connect each PC to the pump and fans via each relay.
The relays need to be setup so that the contacts are CLOSED when there is no power to the coil.
Then connect the coil of the first relay to the output of the second.
And connect the coil of the second relay to the output of the first.

Its important that the coils are connected to the outputs, not the imputs.

That way, if you turn on one PC the opposite relay is opened and the other PC is disconected from the circuit.

As the coils are connected to the relay outputs whichever PC you turn on first will run the pump and fans.
If you have both PC's turned on and you turn off the PC powering the fans both relays will switch and the other PC will take over.

For safety you can put a couple of diodes in the 12v lines from each PC.

I will delete my first post.