Could it be you bought ToV off of steam?
I have vanilla and OF on disc, and bought ToV on steam. Not sure how I'm able to play, seeing as steam usually requires you to buy the base game off them if you want any expansion, but I am.
Got the same error message when installing EIRR though... =/
EDIT:
Since this post turns up first in the search for the problem, anyone else troubled by it should check out
this.