The easiest way to do this is to rename the \Softrak\Ledger folder, and re-install Ledger from scratch. It will prompt you to register, and you can fill in a new registration form and fax or email it to Softrak.
I don't work for Softrak, but it is pretty safe to assume that Softrak won't let you re-register unless it is convinced that you are not violating the terms of the license agreement.
Steve