Sorry for the late reply, but I had to come up with the best way to do it.
1. Back up all important documents and apps from both OS's (including your iTunes library
2. Plug in your computer
3. Insert your OS X install disc when in OS X and reboot from it--it will take a while to boot.
4. Open Disk Utility under Utilities on the Menu Bar
5. Click on the top level of your hard disk--Not Macintosh HD or Windows, but the name of your hard drive (mine is 232.9 GB WDC WD2500...)
6. Choose the partition tab
7. press the plus under the graph of your partitions
8. Rename the new partition on the right (When the drive is formatted to FAT32, it will always be all capitals)
9. Set the format to FAT32 if available (if not I will get to how to do this later in step 9a)
10. Set your size to a bit bigger than your iTunes library (mine is around 85gb, so I would make mine around 95 to 100) and remember that you may not be able to expand it later.
11. Press Apply and wait until it is completely finished.
12. Reboot into OS X.
13. Make sure your files are intact.
14. Reboot to Windows
15. Make sure your files are intact.
16. Go back into OS X
(9a.) When in OS X, open disk utility and select the new partition
(9a2.) Click the Erase tab
(9a3.) Change the volume format to FAT32
(9a4.) Click on Erase
17. Copy your iTunes Library Folder (Usually the Music/iTunes folder)
18. Open iTunes in OS X while holding option
19. Select the iTunes Library file under iTunes on your new partition
20. Your music and playlists should all be preserved
21. Go into iTunes's preferences
22. Click Advanced and make sure that that the path shown in the first box is set to the new partition--if it is not, change it to that folder
23. Make sure that the Keep iTunes folder organized and Copy items into iTunes music folder are checked
24. Repeat steps 18-23 in Windows, but when you open iTunes, hold Shift, not Option.
25. Tada! You are done! I reccomend doing syncing in Windows so that your contacts will always sync right, but you can now add songs and listen on both sides.
If you have any more questions, please ask! I hope this guide helped!