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Step 1. Quit iTunes.
Step 2. Open up Terminal (Applications>Utilities>Terminal).
Step 3. Paste the following into Terminal:
defaults write com.apple.iTunes allow-half-stars -bool TRUE
Step 4. Hit enter.
Step 5. Open up iTunes and you now have the ability to drag the rating to the spaces in between the normal stars.
I haven't tried this on a PC, but it's apparently possible there as well. Follow the source link below for a guide and leave your comments below if you get it to work.
(via Instructables)
I hate that Windows makes you start editing XML to do this. Why can't I just open a command prompt, point it at %appdata%/itunes and have it do the work for me.
@Marc Did you get it to work in the end?