If you registered the Mbox, Digi Customer Service would be able to give you a replacement authorization code.
If you registered and received an amnesty upgrade to Pro Tools 7, or purchased an upgrade to Pro Tools 7 on the DigiStore, you would be able to access your code from the website...
See this Answerbase item (linked from the Download Help quick link in the support section)
Where Can I Find My Pro Tools LE 7 Authorization Code (Serial Number)?