I know how to get thief working on windows 7, but I know nothing of the demo. I say find the full game and follow these steps:
1) Install by running
setup.exe -lgntforce to bypass the "This won't run on windows NT" message
2) Right click on Thief.exe, selected Compatibility, and Run As Administrator and XP Compatibility Mode.
3) Start Thief up, opened options menus and quit, just to generate config files.
4) Unpack DDFix into Thief folder (for me, e
games\ThiefG)
5) Replace the stock Thief.exe with pre-patched Thief.exe from
http://www.saleck.net/Downloads/Thie...Fix-Thief1.zip It's necessary as the DDFixGUI does not do very well with 64bit windows. Manually patching the .exe with a hex editor is possible, but this was much simpler.
6) Right click on the new Thief.exe, select Compatibility and Run As Admin and XP Compatibility Mode again.
7) Unpack
http://www.filefront.com/13839558/Da...creen-v6r3.zip into Thief folder and rand DarkWidescreen.bat, setting up the resolution (1680 by 1050, 100 FOV)
8) The game should work and even show the cutscenes.
It'll be a pain in the ass, but that's how to get thief running on a modern computer.