You can only effectively use a TEC with water. If you use a peltier with air, you are likely to encounter catastrophic results. If you dont cool the hot side enough (Air cooling is NEVER enough) it will heat up the cold side. Things will get worse like a domino, and you end up screwed.
Even if you set up a TEC system properly, you have to be careful about condensation. They can cause ice cubes to form and melt within your computer, which is obviously a bad thing.
People look at a TEC and think it is a simple solution to a really cold computer, when in actuallity it is potentially quite dangerous for components, and overall it costs alot of money to run one power wise.