I was wondering if there is a way to get the briefing and intro cutscenes to work on Windows 7 without having to download the big mod, mainly because I want to play the original game as it was made by the developers, and I heard the mod changes some things around. Right now the cutscenes don't even load at all and just skip when I click on them. I tried looking up but most people didn't have a definitive answer and the ones that did said through the big mod that also changes other things.

Try re-registering lgvid.ax :
- open the command line interpreter (cmd.exe)
- type the following :
regsvr32 path_to_your_thief_folder\lgvid.ax

You should get a confirmation saying something around the lines of "DllRegisterServer succeeded for lgvid.ax"

reboot the computer.

also, TFix (assuming that's what you mean by "big mod") fixes things and makes subtle improvements, it doesn't change gameplay mechanics* or do anything that would affect the gameplay in some inappropriate way. it's not really a mod, it's more of a patch - trying to continue where the official patches have left off. also, you already are using its lite version anyway - the GOG updates are all TFix derived.

*the exception are the interactive candles - while there is quite a bit of evidence in the editor that LGS were trying to go with something similar and then abandoned it because of time or resource constraints, there is no way of telling how would the finished setup work and look, so I had to go with my best estimate. it turned out pretty well though, if I can say so myself.

Thanks everyone, luckily the fix was as simple as running the game as administrator. Here's to enjoying the rest of the game now that I've made it to mission 6 after spending nearly 4 hours on mission 5.