I can't speak for all games, but I do know for a fact about cheating on halo. The only cheats you can do are the ones that don't modify the physics of the game. For example, you cannot use a trainer. When it's on, the game will run fine, but as soon as something that was supposed to happen doesn't (ie a warthog runs you over, but you don't die, or you jump too high) then the game forces the other connection to quit and it goes out of sync and goes to a "Game Ended in a Draw" screen. You can use a glass walls hack, which modifys the walls on your local map to be seethrough (caves, pillars, everything). This is how some people snipe you instantly when you come out of somewhere, but its generally harder than easier to use. You can also do amusing mods on your end, like making everybodys heads big, or applying custom skins to the warthog, but the other players will see neither of these. If you have two boxes however, with the same mod, they WILL work. Both must be using the same ppf or trainer though. So in general, no you cannot cheat on halo. People who you think are cheating are just better than you, and are very amused when you say they're cheating, because chances are they know its not possible.