Highly recommend that you look at this thing....
http://forums.worldofwarcraft.com/thread.html?topicId=62193878&sid=1
Basicially, make back-ups of the folders in your C:\Program Files\World of Warcraft\Patches directory. Specificially the 2.0.0 to 2.0.3 patch. Put the back-ups somewhere else on your hard drive other than the WoW folder, and put'em back as soon as you install TBC. Run the "updater" executable that's inside the 2.0.3 patch folder before you run TBC and everything should be just... peachy. :P
Basicially, make back-ups of the folders in your C:\Program Files\World of Warcraft\Patches directory. Specificially the 2.0.0 to 2.0.3 patch. Put the back-ups somewhere else on your hard drive other than the WoW folder, and put'em back as soon as you install TBC. Run the "updater" executable that's inside the 2.0.3 patch folder before you run TBC and everything should be just... peachy. :P