Hello Robheldreich,
Thank you for using Apple Support Communities!
It sounds like you took the correct steps to de authorize other computers but one is still saying that you have 5 computers associated.
First if you have not already, I would verify that the computer that is refusing to authorize is using the same Apple ID.
If so then I would sign out of the iTunes Store, restart the computer, and then sign back in.
Choose Store > Sign In (or Sign Out).
From: iTunes 11 for Windows: Manage your iTunes Store account
http://support.apple.com/kb/PH12507
All the best,
Sterling