Fanboi primer: How to move your iTunes from PC to Mac
Step-by-step instructions for Jobsian surrender
It's all relative
The first step is to save the mp3 files to a hard drive that's accessible by the Mac. Because I was using a Mac Book Pro, I copied the entire contents of my old Music folder to an external hard drive. This allows the new system to use the same relative paths as the old one.
The Windows files that store most of the data in the iTunes library are iTunes Library.itl and iTunes Music Library.xml. In Windows XP, they're found on in the My Music\iTunes folder.
Amazingly, there's little difference between these files as they reside on Windows and OS X. Our work involves making a few minor modifications and then copying them to the right Mac folder. The first change we'll make is to the text in iTunes Music Library.xml that shows the file path of each song. On my XP machine, music lives in a folder called Music that's on the M: drive. The file path for the first song in the library is: “file:\\localhost\M:\Music\A.C. Newman\The Slow Wonder\A.C. Newman---The Slow Wonder---01---Miracle Drug.mp3.”
On the Mac, music will live in a folder named Music on the external hard drive. In iTunes, the path of the same A.C. Newman song would be “file://localhost/Volumes/Mac Media Drive/Music/A.C. Newman/The Slow Wonder/A.C. Newman---The Slow Wonder---01---Miracle Drug.mp3” (note the "/" instead of "\").