When you install CRM 3.0, the installation logs are written to a directory under the installation user's Documents and Settings directory:

C:\Documents and Settings\[user]\Application Data\Microsoft\MSCRM\Logs

You should make a backup of these files because they contain a ton of information related to the choices made during the setup process. 

Basically, every question you answered during Setup, including the product key, is written to those logs and should you ever need to recover or recreate an installation, that information will come in handy.