I don't think a OS re-install is required these days to fix problems. LOTRO can run terrible for a lot of reasons, some reasons have no answer it seems.
For instance only several weeks ago LOTRO was running at < 10fps for me. (normally runs at 70fps - thats where i cap it under troubleshooting to reduce power usage) I had not done anything diffferent, but it was almost impossible to play. I tried to fix the problem, rebooted, changed graphics options, restart,restart.... updated drivers etc etc... nothing fixed it other then dropping to 800x600 which got me ~40fps. The next day or so there was a LOTRO update/patch and it all worked perfectly again. I still dont know what was wrong but i swear it was nothing at my end.
but if you want some advice or something else to check, have a look at the userpreference.ini under "documents\The Lord of the rings Online" folder. it has some tweaks that can fix performance issues... one to note is the annoying thing where LOTRO can force windows to drop back to basic theme rather than Aero. This can play havoc on the overlay of LOTRO in windowed mode especially when you run other overlays on the PC, you know those volume control programs that pop up on screen and things like msn messenger that can display over the top... the problem is i forget which setting it is exactly.... but i will quote mine below and highlight the lines i think that can make things better.
By all means compare it with yours and change the settings i have put in bold. it doesnt break anything but can fix some issues. anyhow hope you sort it out... to leave the game because it doesnt work isnt much of a happy ending...