Is there enough free space for your music library? There's a setting in the iTunes preferences that means when you add music, it adds (copies) the files to it's own location. Perhaps there's not enough space on the hard drive to effectively have your library twice over.
FWIW, I'm adding my 70GB of music to iTunes on an XP machine right now. I simply dragged my music folder into iTunes and it's working although it can take a while to respond.
2nyRiggz's idea won't add the music to the library.