You might also try running the detect and repair in Outlook. It may be a corrupt calendar entry that is hosing you. I also have been using Office 2003, including Outlook 2003, with my 4155 and 2215 since Beta days. Have a regular retail version of it now and no problems. If you've tried this with several PPCs, I'd be looking at the common item, the desktop.
Sometimes you are the anteater, sometimes you are the ant.