There's already a tip that describes how to add half-star ratings in iTunes 8 for Mac users: http://www.tipstrs.com/tip/7346/half-star-ratings-in-iTunes-8. But what if you are using iTunes on Windows? Yes, you can get half-star ratings on the Windows version of iTunes too, here's how:

1 - You're going to need to edit the iTunesPrefs.xml file, so your first job is to find it. Probably the easiest way to find it is by going to Start -> Search and just searching for the file by now. Once you find the file, open it in your favorite editor.

2 - Find the following lines in the file:

<key>User Preferences</key>
<dict>

3 - Below these lines, add the following:

<key>allow-half-stars</key>
<data>
dHJ1ZQ==
</data>

Save the file and then restart iTunes. You should then be able to give songs half-star ratings.