Should be easy. In XP, go into your Control Panel, navigate to "Other Control Panel Options." You should see an E-license control folder. Keep navigating into the folder until you get to the e-license utility for OOTPB 2006, and run that. As long as you're connected to the 'Net, it should be automatic.
It's a good idea to write your license number down before you unlicense, so that you will have no trouble re-licensing later.
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