TuneSync
TuneSync is a Windows application that transfers ratings, play counts and last-played times—as well as other song data—between iTunes libraries. Use TuneSync to keep all personal information about songs synchronized between two or more copies of iTunes. TuneSync should be run every so often between the same libraries to make sure that they all have the most up-to-date information, regardless of where you’ve made the changes.
TuneSync loads two iTunes library files, compares them using various heuristics and lets you synchronize selected information between the two. You can then store the changes to both files and force iTunes to reload its metadata from this library. You are in full control over the information that is synchronized from one library to the other and vice versa and you can even edit information directly if neither side is 100% correct.
You can download the Windows-only software for free or browse the extensive documentation and tutorial.