This is actually easier than you might think. I did some experimenting via Parallels. All you really have to do is copy the files in user/Music/iTunes on your Mac over to My Documents/My Music/iTunes on your PC. (You can just delete the ones that were there by default.) The files are compatible, but of course the file location paths are going to be different from one computer to another. When you start up iTunes on the PC it will update your files so that their paths start with My Documents/My Music/iTunes/. This works even if the file was on an external hard disk originally, but you'll have to copy the actual music files to the iTunes Music folder on your PC.
For example, I tested this out with a 10,000 Maniacs album. On my Mac, the path listed for "These Are Days" in the iTunes Music Library.xml file is:
<key>Location</key><string>file://localhost/Volumes/Macintosh HD/Users/icruise/
Music/iTunes%20Music/10,000%20Maniacs/10,000%20Maniacs%20MTV%20Unplugged/01%
20These%20Are%20Days.mp3</string>
<key>Location</key><string>file://localhost/C:/Documents%20and%20Settings/
icruise/My%20Documents/My%20Music/iTunes/iTunes%20Music/10,000%20Maniacs/10,000%
20Maniacs%20MTV%20Unplugged/01%20These%20Are%20Days.mp3</string>
You can see that iTunes basically just took the Artist folder information and put it in the iTunes Music folder on the PC. However, this only seems to work for files that were in the folder designated as your Mac's iTunes Music Folder (whatever drive it may be on). I'm not sure about how to handle music that is strewn over many drives.
Does that make sense?