Ok, i finally solved this annoying problem.
It's been a long time since i performed maintenance on my PC so i did it last night.
Fresh install of windows xp pro, English version in a clean, freshly-formatted HD.
I messed up a bit with regedit, got the Administrator account visible in the login screen, logged in as admin, installed every update and then the game.
I don't know if it's the English version of windows instead of the Greek one i was using, or the fact that i changed the privileges in the LOTRO files, but everything worked.
UI skins, plugins, everything.
So, after this i can only assume that either the Greek edition causes problems with LUA and UI skins, or it's windows XP Home edition and the privileges, that mess up with the .tga files.
I'm glad i got it to work and i hope NektariosZ will read this and try the same method!
You guys helped a lot by explaining to me how LUA works.