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I haven't used 2007/2008 extensively, but in the past, what you typically do is change the document settings to your liking and then save it as a template, overwriting the Normal Template (normal.dot) file.
 
This is in hopes that you haven't found it yet.

Go to Format --> Document.

Change the margins to what you want and then click the Default button on the bottom left.

This will overwrite the Normal Template (normal.dot) file.
 
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