(1) If you are planning to use the term “kryptonite” in commerce as an identifier of the source of goods or services, then you should consult a competent attorney licensed in your jurisdiction, not a message board.
(2) As Jayrot said, it seems from your description that you are talking about trademark issues, not copyright issues. Single words, names, short phrases, and titles are not protected under copyright law, but might be subject to trademark law.
(3) Take a look at this for some basic information about how trademarks work – http://www.straightdope.com/mailbag/mcopyright4.htm
(3) “Public domain” is a copyright concept. The rough equivalent of that concept in trademark law is “genericness.” You would have to ask whether “kryptonite” is a generic term for anything.
(4) A quick perusal of the Patent and Trademark Office’s Web site shows that there are several registrations for terms that include the word “kryptonite.”