To create an unattend.xml file for sysprep, use Windows System Image Manager:
Click File / New Answer File...
Open the Components box in the "Windows Image" lower left-hand pane of the WSIM window.
Locate and right-click on the blue box labeled "amd64_Microsoft-Windows-Setup_6.1.7600.16385_neutral" if you're running 64-bit Windows 7, or "x86_Microsoft-Windows-Shell-Setup_6.1.7600.16385_neutral" if you're using 32-bit Windows 7.
In the drop-down menu that results, choose "Add Setting to Pass 4 Specialize."
To the right and up, under the "Properties" section, click "CopyProfile."
In the small down-facing triangle that appears just to the right of CopyProfile, choose "true."
The CopyProfile setting will tell Windows to copy Testuser's profile to the Default User profile when the prototype image (which we're about to create) gets deployed to a system, which is how to change the default Windows profile.