Via Lifehacker..
If you live in a cold area, there's no reason to buy expensive and complicated cooling setup for your PC. With just a bit of dryer ductwork, you can actually cool your PC using air from outside.
While most computers don't need extra cooling, if you've overclocked your PC for video editing, gaming, or other processor-intensive tasks, keeping your computer cool is a must. In fact, the better your cooling system, the higher you can overclock, so a bit of winter air is sure to provide some nice speed boosts.
All you need is a flexible duct, some pieces to connect it to your window, and some scotch brite to act as a filter. By setting up one of your PC fans near the duct, you can blow air from outside right into your computer case. Note that the above setup, while clever, will probably cause your heating bills to go up as the cold air is funneled into the room through the PC—if you set up a second duct with a fan blowing the air outside, you might make this a bit more efficient. Hit the link for more details.
More pictures here.