Provided by Dennis Passmore in the Delphi Newsgroups:

 If a person would just backup there (sic) "C:\Documents and 
 Settings\{userid}\.borland" folder after they have register 
 Delphi 2005 and before a uninstall then all they have to do
 is restore it after a re-install it and then there is never
 a reason to re-register as long as the user never changes 
 there "user name" on the PC since the registration 
 information is directly linked to the login user name.

NOTE: This only works on a system with the same hardware because this activation information is specific to hardware.