Welcome to TTLG . It seems to me that you're using an original (DRM-) version of Thief 2. Perhaps you would wish to patch it with DDFix in order to resolve some nasty graphics (like 16-bit dithering and a fog absense) and a multi-core issues, but you must follow the FAQ to make this. If you wish to solve just the multi-core issue, you may try this tiny and pretty simple solution.
P.S. I think you should have post your problem in another topic.