The Office 2007 Trial will not Upgrade your Office 2003. You do not need to
uninstall it but make sure when you do install the Trial install it to a different
directory than the Office 2003 or otherwise you will have a surprise when trying to
uninstall or install Office after.
You can of course uninstall Office 2003 which would be your safest bet before
installing the Office 2007 Trial