Windows Small Business Server 2008 supports installation using an answer file.  The answer file provides the following functionality:

• Automates the SBS 2008 installation process for both a clean installation as well as a migration.
• Allows you to run a clean installation setup in advanced mode interactively.

In addition, the answer file is the only way to to run the windows SBS 2008 installation in join domain/migration mode, there is no other way to trigger the SBS 2008 setup to act in that mode.  Additionally, if you do not want to use the default of .local as your TLD, you must use an answer file to specify an alternative TLD.

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