Don't force people to be in Admin mode, for a start that instantly rules out most people trying to run it in at school/uni and maybe most non-technical job networks.
Dunno about GameMaker-specific issues, but the correct directory to put application ini files etc in is given by CSIDL_APPDATA (there are a lot of these for various folders, see
here). That one (when passed into some Windows API function whose name might occur to me) will get you "C:\Users\<user>\AppData\" on this PC.
This post might give you some clues... gives examples in a few languages, and also the "wrong" way of getting it from an environment variable which might be a decent fall-back option in GM.
http://stackoverflow.com/questions/499095/best-way-of-getting-path-to-application-data-directory