From: "Rich Gauszka" 

An attribute of the Vista clean install process that seems worthwhile -
details at link

http://support.microsoft.com/kb/927522/en-us

You can perform a clean installation of Windows Vista on a hard disk
partition that contains an existing Microsoft Windows installation. If you
do not format the hard disk, all the following items will be saved
automatically in the Windows.OLD folder after Windows Vista is installed: .
The previous operating system
      . User data
      . Program files

....

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