The game loads up perfectly fine but when I try to start any missions the games crashes right away. I've tried switching to a different compatibility mode, run as administrator etc... Any ideas guys?

tried the sticky?

Try going into your Thief directory and opening ddfix.cfg and see if the line MultiCoreFix is set to 1 if not then set it to 1.

If that's not the issue then may I ask if you can see the opening intro? Now dark engine games skip the intro after the first load up so just click the intro button and see if it plays or the game just crashes if this happens then it's because the movies aren't playing properly so try registering the indeo codec by creating a batch file in notepad called RegIndeo.bat in your thief directory with the following text

regsvr32.exe /s LGVID.AX

now just double click on that and hopefully that should correct that problem.

or try Run as admin command line then typing these two commands:
cd c:\windows\SysWOW32 or Syswow64 if your running 64 bit windows
regsvr32 ir50_32.dll

Also when you installed Thief did you tick the box in the installer marked "Register Indeo Codec" ?

I'm having the same problem. I've tried everything - Administrator privileges, ddfix, Compatibility settings - nothing has worked so far. Help, plz?

you should post your full system specs and make clear whether you are using tgfix or not.. anyway, renaming LGVID.AX (this disables the cutscenes) is a good start for anyone who has a crash right before the game starts.