I could be mistaken, but it looks like someone managed to randomly generate your CD Key. Try asking in the Technical Support Forum, they could point you in the right direction about having this rectified. Especially since your key is registered.
Your post did not specify if in use by you or by someone else. If the key says in use by you then proceed to the directory Diablo II was installed (Typically c:\program files/Diablo II and delete the bncache.dat file.
If the key is in use by another account please see: http://us.blizzard.com/support/article.xml?locale=en_US&articleId=20633&parentCategoryId&pageNumber=1&categoryId=2616
3. Re: CD KEY in use by : key 07/17/2011 03:40:24 AM PDT
The best idea now is to use that link Bobertith has in his post. If you're the legitimate user of that key, some idiot has either keylogged you and stolen it or some program (likely from an item site) has generated your CD Key and is using it to farm or spam.
Good luck, and I hope they get it resolved for you.
Everyone and their brother wants to have a couple angelic sabers.
7. Re: CD KEY in use by : key 07/21/2011 03:36:34 PM PDT