I will quote a fellow MVP (Neo):
"Uninstalling Office/Outlook 2007 will most likely leave behind
a newer MAPI32.DLL that is not compatible with Outlook 2003.
All you have to do is rename/delete said file @ \Program Files\Common
Files\System\MSMAPI\<LANGID> and then do a detect repair
of Office 2003 and you should be good to go."
Nikki Peterson