I'm getting the message "Error: Game not installed." When I try to run the graphics updater (No Remorse).

I also tried editing the .conf files for DOSbox directly, but it wouldn't allow me to save the file after changing it, for some reason.

I'm running 64 bit Windows 7. I have tried running the file in compatibility mode, but it hasn't helped so far. The game (and all my GOG.com stuff) is installed under the Program Files (x86) folder and requires admin permissions to change anything - which I'd like to change anyway - anyone know if that would help?

Yeah, definitely reinstall to a folder that's not in Program Files. After that, run the graphics mode setup in the start menu rather than the game folder and it should work for you.