If that is the case, then one could easily edit all of them to the correct data.
So, if on Windows it is
C: \Users\Bob\Documents and Settings\My Documents\My Music\iTunes\iTunes Media\...
and on OS X it is
file://localhost/Users/Bob/Music/iTunes Media
Then you can open the XML file in a text editor, and tell it to find "C: \Users\Bob\Documents and Settings\My Documents\My Music" and replace it with file://localhost/Users/Bob/Music
I would highly recommend backing up the original XML file.
(I am not currently in a position to test this theory, my PowerBook, for obvious reasons, does not have Windows on it.
Conversely, do this
Copy across your library file Now find the iTunes Library.itl file on your Windows PC (it should be in the My Documents My Music iTunes folder). Copy this across to your Macs iTunes folder (note: this is not the iTunes Music folder but is usually one folder back from that you have the correct folder if it has the iTunes Library and iTunes Music Library.xml files in it). Now, delete the iTunes Library file on your Mac, and then rename your newly copied across iTunes Library.itl file to iTunes Library (i.e. remove the .itl from the end). This has given your Macs iTunes a new library file pre-populated with all the extras you wanted to keep from your old library.