I have had constant crashes, finally solved the crashing on starting the mission by starting in administrator mode but still having trouble at times when loading and such. If I run it in compatibility mode, with XP service package should I do compat mode for? One more question, any good fan made patches out there? --- Get out of here stalker.
If running the game as an Administrator seemed to help, let me ask where you installed the game. Did you install it in the default folder (probably C:\Program Files)? If so, you might want to try installing it elsewhere in its own folder (e.g. C:\ThiefDS) and see if that helps. Windows puts special protections on some of those folders.