I would ensure the trial version is properly uninstalled in Vista's Control Panel then Progams & features, then I would uninstall Office XP using that Office XP CD.
Then, reboot the computer, (restart it). This allows the system to update itself properly. Then try installing Office XP again.
I had successfully installed Microsoft Office XP Professional on a Vista Home laptop with pre-installed Office 2007 that was never activated. To avoid 2 problems that repeatedly occurred: (1) Auto updates that crashed because the trial Office program wasn't activated, and (2) To open a file, the default was set to trial Office program so that I had to choose XP Office first in order to open any file - I went to Microsoft website and used FIX-IT feature to delete Office 2007. However, it seems to have also corrupted Office XP. I tried to reinstall it, but it gives me an error that it cannot find the file inkobj.dll? Where do I find this file in order to fix the installation of XP Professional? This is for personal use on a very infrequent basis, and so I don't really want to spend money on new Office software.