I thought my new P4 - 1.4 GHz processor was running hot.  After I built this cooler, I figured out that it was not running hot.

I installed Motherboard Monitor, and had the temperature sensors wrong.

My system temperature runs around 59 degrees (F) with the vent and 95 degrees (F) without the vent.  That is a drop of 36 degrees (F).
The CPU temperature runs 37 degrees (F) with the vent and 86 degrees (F) without the vent.  That is a drop of 49 degrees (F).

Materials

• 8'x3" aluminum flexible exhaust pipe for a dryer - $14
• hood with bird/rodent guard and 3" pipe - $6
• duct tape - $2

• some way to drill a 3 inch hole in the wall (drill attachment)
• tool to drill 3" hole in side of computer case (suggest Dremel tool)

Create a hole in a window or the side of the house.   Run the hood/pipe through the wall.   Attach one end of the aluminum flexible pipe to the hood pipe.  You can attach with duct tape.  Cut a hole in the side of the computer case, and run the aluminum pipe inside the case so that it is positioned above the CPU.   The CPU fan should draw the air into the CPU heat sink.   Since this house is not well insulated, and there are cracks, heat will generally be pulled out of the house, which means that air is drawn in through the pipe (this is a good thing).  If the house was well sealed, or there is some way for the house to be pressured positively, heat would be pushed out through the pipe, and no cooling would take place.  Make sure to seal all holes in computer case to prevent the power supply fan from pulling air from the room.  We want the air to be pulled from outside.  My temperature measurements are taken without the cover on the computer case, so the temperature decreases are probably not as low as what they would be with the cover put back on.  Actually, we don't want to cool the motherboard down below the freezing point of water.  The CPU can operate below that temperature, but the capacitors on the motherboard contain water, and will freeze and destroy the capacitors.   Motherboard Monitor software has an alarm for when the temperature goes below 41 degrees (F).   Some of the holes in the case should be left open to keep the computer from getting too cold.  The other option would be to isolate the CPU from the motherboard using a vent that goes out of the case.   This would allow the cold air to only affect the CPU and not the rest of the motherboard.

Disadvantages:

• Need to live in a cold climate.
• This technique will not work well in the summer.  Since most of the year in Vail, CO is very cold, this is not a problem.   Summers only hit highs of 80 degrees or so.
• Smells - I can tell when a snowmobile drives by, or when neighbors are having a fire in their fireplaces.   The fan draws in all sorts of smells.  It is kind of funny, because the smells are coming from the computer.   Adding a filter may help prevent this.