How can I keep a PC dry outdoors?

I'm planning on having an outdoor LAN-party, not in the rain but you know how water condenses on everything.

The best way to avoid getting moisture in your computers is to use a desiccant pack. Probably the best material to use for this purpose is a moisture absorber like that found in a modern diaper. As long as you protect the computer so that no water can directly run into the case, any condensation that occurs will absorbed by the diaper. Placing this item inside the case and covering the CPU enclosure with a plastic tarp should do the trick. I have operated equipment outside like this (in a rainstorm no less!) for several hours without any trouble.

Water condenses when it becomes colder, and a running computer is very warm.

The thing you want to look out for, is bringing a computer from an air-conditioned environment (indoors, from a cold car, etc) to a warmer moist environment. Humid air will hit the cold computer and condense.

What you should do is make sure the computer is about as warm, or warmer, than the surrounding environment before you turn it on, especially when the humidity is high. Steel and aluminum computer cases always feel colder than the surrounding air if they are at the same temperature, but keep the temperature difference in mind.