So whenever I play with one of my friends, 1p uses keyboard and 2p uses keyboard+mouse. I've noticed that whenever one of the players presses more than two keys, the other player freezes up. For example, if 2p starts crawling around, 1p can't move or do anything at all. When 2p releases one of the keys, 1p can control the actor again.
Does anyone know whats going on?

Pardon me if this has already been stated, did a search and the only thing I found related to two player errors was the sharp-aim glitch.

This is caused by your keyboard being unable to queue and send more key press and release events.

One option is to try using different keys.
Second option is to get a better keyboard, preferably a gamer one, that can handle more keys at once.
Third would be to use two keyboards.
Fourth - you could use a gamepad(s).

As asmagedon already said, it has to do with your keyboard. The phenomenon is called ghosting. Depending on what keyboard you use this either means it will either lock one more keys or give a signal for a key that is not pressed, when you press more then 2 keys.

This should be solvable by making sure that the keys you and your friend use are in different "sectors" of the keyboard. The problem is that these sectors are hard to find.

Does this mean you can't even press Ctrl+Alt+Delete?

Most keyboard manufacturers are smart enough to make sure that common combinations are usable.

One should hope so, anyway.

Seeing as I'm playing on a little laptop right now, the keyboard thing makes sense.
btw, what keys should i use, or will they all have the same problem?

There really isn't much choice beyond trying a lot. The keyboard manufacturer is unlikely to release info about the keyboard's zones. Your best bet is to plug in a better keyboard.

Or do what I did and get a numpad extension. My laptop doesn't have one at all.