Hi! I've installed a mod to replace the common houses interiors, but I want to use the original ones. I have backuped the original texture folder, but I can't replace them with the original ones.....what can I do?

Delete the new textures. Morrowind will then automatically revert to the textures in its *.bsa archive.

Mr. Swiveller is correct.
An easy way to do this is to take the textures from the mod and copy-paste them into the Data Files/Textures directory - they will all then be selected and you can delete them with one fell key-press.