Halo for PC is protected by Safedisc 2.70.30. I am assuming you are trying this on the original uncracked exe. One of the features of safedisc is to fill portions of the code with int 3's, and then have a debugger run parallel to the game process and replace the int 3's with the original game code. Since this safedisc debugger is running with the original exe, you cannot attatch another debugger because 2 ring3 debuggers cannot debug the same process. I think you may have some more success with Softice, which is a ring 0 debugger.
I think that this may be the problem. Try coding the trainer using the cracked exe. I think that the memory locations and all that other crap are the same in the original and the cracked exe, so a trainer for the cracked exe should work on the original exe and vice versa. I am not sure about this, as I do not have a lot of exeperience with game training.
Hope this helps
