I am planning on making a cooling device for my computer:
All you need is a bucket made out of polysterne, some dry ice, water, some kind of tube and duct tape.
You put the dry ice in the bucket, add some water, ducttape the tube on it in a hermetic way, duct tape the other end of the tube to the computers main fan (next to the power supply for me,dell dimension 8200) and you have a cheap and effective cooling device. It should work pretty well...at least in theory.

If you have some beckton and dickinson lab tubes, put a little bit of dry ice in them, add about twice as much water, and close it down. Take cover and wait until the thing explodes