I had a problem with running LOTRO under Windows Vista.
For some reason I got a message saying that the game couldn't open the data files. Had a look at the FAQ on the LOTRO website which suggested it was either because I wasn't logged on as an administrator (which I was), or the files were corrupt so uninstall and try re-installing.
Instead of uninstalling and then re-installing the game, I decided to see if switching "user account control" off in Vista did anything... and it did! I don't understand why this should solve the problem, because I am the administrator on my computer and no-one else uses it. Anyone out there who might know of a reason why?
Anyway, it's not really a problem, I just run the game with user account control disabled.
So if anyone else has a similar problem with Vista and LOTRO you might try my solution.