ThiefFanatic, I had that problem too with Thief 1 (Gold) where it would freeze after the mission briefing. To fix that right click on the Thief Icon - Click Properties - Click Compatability - Under Compatibility Mode click in the check box and choose Run Under Windows 2000. That should clear that problem up.
Then I had that problem with briefings not playing once I fixed that. And to get the videos to play there was a fix that works beautifully, it has to do with finding some file, copying it to the Thief Installation Folder, and creating some text document and save it as something. Everytime the videos don't play just exit the game, double-click that created text file and they will run fine after that.
I had no problems running Thief 2 on XP aside from the videos not playing which was fixed with the above fix.
So yeah Thief games can play on XP, but they really really don't like that operating system. You pretty much have to trick the games into thinking it isn't XP.
Edit: All these tips and much much more are talked about in the Thief Technical Forum. There are like hundreds of will Thief run on XP threads.
"What is locked can be opened, what is hidden can be found, what is yours can be mine."