When moving from mac to windows:

1) copy the iTunes folder across
2) find the file called "iTunes Library" that has no extension. Its in the same folder as a file called "iTunes Library.xml".
3) Copy this file in the same folder.
4) name the copy "iTunes Library.itl" in other words give it a file extension of "itl"
5) Find your iTunes icon on your desktop
6) hold the shift key down, do not release it
7) double click on the icon using your mouse
8) iTunes will ask you to use a different library
9) browse to the folder containing your itl file created earlier and select the file
10) voila - your mac library is now working in windows.

When transferring from PC to MAC do the above procedure I gave in an earlier post in reverse - ie you just remove the ".itl" extension from the "iTunes Library" file.

Also in order to get the MAC to ask you for a new iTunes Library you must Hold down the option(alt) key when starting iTunes (sometimes to get this right you first select the icon, hold option, then click to start it).