If you'd looked in this thread you'd have found what you need to know.
I actually installed TG on Win7 this morning.
OK, Put CD in drive -
Go to Start, then Run, then type d:\setup.exe -lgntforce, where d:\ is your CDrom/DVDrom drive.
Install, run it (to set up the cam.cfg file), it will probably freeze and you'll have no vids.
Close it down.
Extract DDfix to the directory your Thief.exe is in.
Run the ddfixGUI.exe (right click, run as administrator), check the video fix box and choose what resolution you want. (if you have a wide screen/non standard resolution monitor, then just leave it on 800x600 and we'll fix it later). Click apply patch.
Go to the Thief Gold shortcut on your desktop or the THIEF.exe file, right click, go to compatibility, choose XP, service pack2 and run as administrator.
There we go.
If you have a wide screen monitor now extract the widescreen fix to the folder with your thief exe in. Run the DarkWidescreen.bat file and enter your resolution. Enter 100 for the fov.
Sit back and enjoy Thief Gold.