FlintDragoon: Windows 7 home premium
Ok that shouldn't make a difference. Heres what I have done just now:
1) I went to Diablo on GOG and downloaded setup_diablo_1.09_v4_(27989).exe.
2) I installed it to the default C:\GOG Games\Diablo
3) I then ran setup from my Hellfire CD (by right clicking the CD clicking open then launching setup.exe
(And installed it to its default C:\SIERRA\HELLIFRE (I also checked Hi-res.) I didn't install any demos, clicked OK to the install message that comes up, clicked OK again, then NO about updates, then OK again, then REGISTER LATER, followed by NO for the read me, followed by FINISH.)
4) Now i'm removing my Hellfire CD.
5) Then i'm copying the HELLIFRE folder from C:\SIERRA to C:\GOG Games\Diablo (so i'll have a sub folder)
6) Then i'm going to go to CONTROL PANEL, PROGRAMS And FEATURES, and remove HELLIFRE and SIERRA UTILITES. (And clicking NO to rebooting when Sierra Utilities asks me to.)
7) Now i've downloaded Hellfire Enhanced and have exteracted the contents (or copied the contents to) C:\GOG Games\Diablo\HELLIFIRE , replacing anything it wants to.
8) I then ran hellfire.exe from the C:\GOG Games\Diablo\HELLFIRE folder and it runs fine.
I've just sat and done all of that under the latest GOG version (I typed this as I went) and it launches fine for me.