When Microsoft released Office 2003, they made it work only with th NT kernel (i.e. Windows NT, 2000, XP). They did this to basically obsolete the 9x kernel and lower support issues and cost. So basically, in order to install Ofiice 2003 the installer must not only detect a NT version of Windows but must also make use of the NT API, which I am guessing has not been fully implemented yet. I'm sure they'll get it some day.
Hopefully Codeweavers comes through on their promise on the "Compatibility" homepage.