Have you checked the file integrity of the game? It is the only thing I can think of
It could also be related to where, in what path, TW3K is installed, I mean if it's installed in the UAC-protected C:\Program Files(x86) folder.
If it's then try reinstall the game outside of UAC and try the MoH DLC again, if that helps.
Under patronage of General Brewster of the Imperial House of Hader.
How to make Morrowind less buggy for new players - Of course every player may find it useful.