I think wildwood is on the right track. Did you uninstall the trial version or just install the full version on top?
Ssometimes the best thing is to uninstall the programme completely and install the latest version.
I also suggest you uninstall the entire Office 20003 application and start again. If you've already done this and still have issues then the only recourse is to follow the link below to the microsoft office help...
http://support.microsoft.com/kb/896301/en-us
Hope you're up and running soon.