This is not recommended for slow computers or a computer that is low on memory. In the console, type "avi record<(filename).avi". You can type in "pause" or "stop" during the recording as well.
Note: If you do record a avi, it will boot you out of the game and it will ask you what program you want to record it with. You have to determine which one is the best for you (which one will use up the least amount of MB's).