Have a look at this helpful page:
http://www.entourage.mvps.org/error/page.html.
It reports that an error #51 is typically associated with your installed version of Office possibly having some conflicting components. This often happens if your Mac came with the Office test drive but you didn't uninstall it before installing a full version of Office.
Regardless, its answer is to use the uninstaller to remove Office and then to reinstall Office. You shouldn't lose any data by doing this.
If this doesn't help, you should also try posting in the microsoft.public.mac.office.entourage newsgroup found on Google or the msnews.microsoft.com server in Entourage.
Hope this helps! bill
1 GHz Powerbook G4 Mac OS X (10.4.5)