First... make sure you installed in this order = Morrowind->Tribunal->Bloodmoon (if you installed TR after BM, a simple re-install of BM will suffice)
Secondly...
and more importantly... this is a http://www.gamesas.com/index.php?/topic/781497-vistawindows-7-problems/page__p__11345653#entry11345653 for anyone using Vista or Win7.
But.. the quickest way to fix it is just by simply http://www.gamesas.com/index.php?/topic/781497-vistawindows-7-problems/page__p__11352753#entry11352753 This will fix this error, and many others that can occur, as well as making mod usage much easier.
:)
edit: fix wording