None, if the full product version is the same version and language as the
trial software. But, for example, if you have the Office 2003 Standard trial
and you want to end up with Office 2003 Professional (which is not available
as a trial), then you should uninstall the trial version, and purchase and
install the full version of Office 2003 Professional. See
http://www.microsoft.com/office/trial/convert.mspx for more information.