Fairly often someone comes up with the idea of using that old mini fridge to cool their computer (guilty of this myself). Unfortunately, without considerable modification to the fridge, there is no way this will actually work. The most common plan is to put a reservoir inside the fridge, and then water cool the computer with water that is chilled by the fridge. Another popular plan is to put the motherboard and its components directly inside the fridge. While this seems like a good idea at first, it just won't work. The reason for this is that the phase change system in the fridge can't cope with the heat load of a modern processor, let alone an entire system.
Lets start by looking at the heat load a mini fridge is usually under. A large mini fridge (3.2 cu ft) has a surface area of around 1.85 meters squared. Its walls are made of 1" styrofoam, with a metal or plastic casing. Assuming there is free convection on all surfaces of the fridge, and the fridge is at 4 degrees C in a 22 degree C room, the heat transfer out of the fridge is around 20 watts. (this value is probably a little high, I haven't counted a lot of the smaller thermal resistances)
When you put something in the fridge, this temporarily increases the load on the system. A good mini fridge can cool several liters of beer....err... water down to drinking temperature within an hour or so. The phase change systems inside these fridges do have the capacity to remove fairly large amounts of heat, unfortunately they don't have the capacity to do so continuously. The biggest problem with a mini fridge is its condenser. The condenser of a mini fridge differs quite greatly from the condenser of a standard phase change system for a computer. In a system made for a computer, the condenser is like a fairly large radiator, with a lot of surface area, and a good fan blowing across it. This allows it to dissipate a large amount of heat, while the fluid exiting it is barly above room temperature. The condenser on a mini fridge is generally just a series of pipes with very little surface area. To make things worse, it is usually built right into the side of the fridge, under the metal casing (feel your fridge when the compressor is on, one side will be warm). This consenser is not able to handle large heat loads for any sustained period of time, and will not generate very low temperatures.
If you have an old mini fridge laying around and you really want to use it to cool your computer, your only real option is to change the condenser. Once you are doing this work though, you may as well just make a direct dye system and get some real cooling going. The compressor from a mini fridge is usually big enough for a DD system, but be warned that these compressors are not made to run continuously so it will need some decent cooling on it, and it probably won't last all that long.
Edit: There are a lot of smaller coolers made for holding a 12pack or some lunch meat or whatever you want, that people will consider using because they are usually very cheap. These don't use a phase change system like larger mini fridges, but use a small pelt. These coolers have a lower cooling capacity than even a mini fridge does. They usually use a small (10-50 watt) pelt, with the hot side passivly (or sometimes) activly air cooled. They have no chance of working, even with considerable modification.
