Heres a way I found to install Thief 2 when you can’t get passed that 100% bit in setup:

Open up disk 1 from my computer and copy all of the folders into a bogus directory eg: c:\program files\thief3. Then copy folders ‘thief2' and ‘editor’ from disk 2 into the same directory. Copying all the folders might take awhile. Do not replace any files when copying from disk 2. Now run the setup from the directory you just copied all the files into. After about 5 minutes the setup screen should come up so click next. If it doesn’t, then run the setup again from the directory. It might take a couple of tries so keep trying. In setup, install all files into any path except the one you just created and click next. Setup now assumes the bogus directory is disk one. When prompted to insert disk 2, just click yes and select your d:\ drive (or whichever letter drive you have placed cd2 in). Setup now finishes from disk 2.

I don’t know the technical reason for why this works but it did for me so.