If I have installed Morrowind+expansions on one computer, can I then copy the programfiles onto another computer that has vanilla Morrowind installed?
This is related to another question I had aske about Tribunal not installing properly. I suppose that one of the important issues is: do the expansions actually changed the registry for the game, or do they just change the existing files within the Program Files? If they only change the files, then this should be possible.

If by program files you mean the expansion program files, then the answer is no. The expansion installation process makes changes to the registry. If those aren't in place, it won't work.