Get Off Those Maliciously Loaded Scripts!
Download this free Anti-Malware Plugin for WordPress.

I’m having this exact problem today. I’m hoping for a solution soon.
I am already registered and one of the sites (site is showing up as one of my registered sites and the key  is correct) is showing that I still need to register. When I click on the Register Now button a new window opens (like a WordPress login screen) showing all the correct information (name, email, site name, key, etc.) – but at the top it says: ERROR03111003: This Installation Key is already registered. Try using the Built-in registration form on the Anti-Malware Settings page in the wp-admin of your own site.NO old_user_email!

Please help.