Should be possible by setting "UnattendMode" to "DefaultHide". From the Preinstallation Reference:
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DefaultHide
Specifies that answers in the answer file are defaults. Unlike UnattendMode = ProvideDefault, Setup does not display the user interface to end users if you specify in the answer file all the answers relating to a particular Setup page. If you specify only subsets of the answers on a page, the page appears with the provided answers. The end user can modify any of the answers on the displayed page.
Edit: hmm, just spotted the following in the reference:
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Important
You must always specify a value for the ProductKey entry.
You may be out of luck after all
This post has been edited by Ctrl-X: 11 September 2006 - 08:34 AM