How do you get the command prompt up to type in cheat codes in Halo 1 or Halo...

[CokoKitten =]]

...Combat Evolved? My brother is desperately trying to get the command promt up to type in his cheat codes but can't remember how; I can't find how to do it anywhere. Help?
Easy 10 points to working answer. =]
This is the PC version of the game, by the way.

Thanks!

 

[lime green luver]

google it and you will probably find it