1920x1080
I can (or once could) set the resolution in game to my monitor's dimensions, but whenever I go to enable fullscreen it just gives me the little 15-second prompt asking if everything is A-OK before it resets. The problem here being that nothing is altered whether I let it count down or accept the "changes." It stays in this borderless windowed mode (minus the bar at the top with Halo 2, a minimise, and an X to close the program.
Now as to why I said "I once could," after finally figuring out that I couldn't connect online anymore because an update had released, I uninstalled, re-downloaded, and updated Halo 2 with the new .exe file. I created an account on the home page for the launcher and even tested logging on. It worked until I tried the DSfix and promptly messed up. So I went through the process about four more times, each getting similar results.
I would: > Install Halo 2 from the .iso > Full install, do not start, > Register later (the last prompt before you're supposed to run the launcher/patcher
After this, I would get the launcher to finish, log in, and Halo 2 would start. However, It would be restricted to the top left in something around 1600x900-ish, run for about three seconds, crash, and then AVG would give me a notification saying that a threat had been removed, the launcher. After that, If I try to open Halo 2 I get the "Live gaming has failed to initialize" error. To fix that I tried installing GFWL, and that would get me as far as the main menu, where a drop-down would ask me to sign in ti GFWL (similar to whenever I logged onto Halo 2 via my 360 and it would sign me in through XBL).
I didn't sign in, uninstalled GFWL, tried one more time, and got the same result as described earlier. It's worth mentioning that prior to trying the DSfix, AVG never interfered with the process. So here I am now, utterly stuck.
P.S. Sorry for the Great Wall of Text