In my quest to achieve local harmony between the computers in my home network, I try to centralize everything using a fileserver (come Macworld 08, I'll likely be changing things up - how depends on what shows up then; regardless, a post for another day). Generally speaking, the idea of having a machine sitting somewhere that I can connect to at will and grab whatever I need works fairly well. It avoids tons of file duplication, is reasonably fast (wired computers see speeds near that of a local drive thanks to gigabit ethernet; 802.11n wireless is generally at least fast enough to stream 1080p h.264 video), and with a bit of router voodoo, I can grab any file I need from any computer with an internet connection.
While my Macs and PCs can by and large coexist peacefully, music via iTunes can get a bit finicky at times. The issue arises in the fact that everything is stored in a library file, and that this file is OS-specific thanks to the different file systems. To make a long story short, you can share the actual audio files between multiple computers, but you can't centralize the library file too easily.
Enter iTunes Folder Watch. It's a bit of a workaround solution, but as the name suggests, it watches a folder for music files and adds them into iTunes as they're discovered. Run that on all of your Windows machines with iTunes pointing it at your central network drive (W:\Music\ in my case), and it will automatically keep your libraries relatively synchronized. Unfortunately it doesn't address playlists or a lot of iTunes-specific metadata such as play count and rating stars, but it at least eliminates the numerous issues you can come across by relying on the sharing feature within iTunes - most notably that iTunes doesn't have to be open on the main computer for this to work.
So a quick tip for you to get the new year started. Expect a lot more on higher-end home networking as the year progresses, as it's been a point of both pain and interest for me. Designing a good server, choosing the right software, hardware recommendations, etc. Subscribe to the RSS feed to keep updated!