My dad's crappy dell has a P4 2.4ghz in it and its kind of passively cooled. There is a large heatsink on the processor and then there is one fan on the back of the computer with a shroud going down to the heatsink.
I have the same thing in my old Dell. Pentium D and all it has on it is a big Heat sink. There is a shroud that directs the airflow from the front of the case under the shroud out of the fan on the back. The rear fan runs all the time and I've been using this computer for at least 4 years now with no issue. I guess you could do it that way, if you are absolutely cheap and looking for a way to make a quick buck on old mobos. Oh wait, your not Dell.
Yes you can!
Put your pc into a larger sized aquarium, make sure that all the wires are fixed well.
Buy 100 litres of sunflower oil, fill the complete aquarium with it and voilà you have a oil cooled pc!
Since oil does not conduct energy it should be possible...