I found a thread on this subject in another site that may help you with this.
view_topic.php?cat=1&otid=14165&forum_name=General&title=Rating%20Sytem%20Help
I have not really looked into it just yet, just bookmarked it for later when/and if I get some time.
hmm, I think the above is a bad link...
here's the direct link
css-star-rating-part-deux/