Microsoft Office - Uninstalling a Trial Version Before Installing the Full Version
You may encounter licensing errors if you install a full version of Microsoft Office 2007 or 2008 on top of a trial version. In order to avoid this, uninstall the trial version of Office 2007/2008 using the following documents:
Once you have uninstalled the trail versions, begin installing the full version. Proceed through the on-screen instructions until installation is complete.
This document explains how to properly uninstall a trial version of Office 2007/2008 in order to install the full version.