Hay Luke,
First of all i would suggest you to take a backup of your database.
Rules, mailing lists, signatures, messages, address book, schedules,
contacts, tasks, notes, calendar are located in a database:
Entourage X and 2004 : ~/Documents/Microsoft User Data/Office X
Identities/your identity
Entourage 2001: <Startup Disk>

ocuments/Microsoft User Data: Office 2001
Identities:your identity
Then I would suggest you to rebuilt your database
a.. Make sure you have enough hard disk space available for twice that
amount. As part of the rebuilding process, Entourage creates a second
database file.
a.. Quit any applications that are running, including virus-protection
applications, all Office applications, Microsoft Messenger for Mac, and
Office Notifications, because they might interfere with rebuilding.
a.. Entourage 2001 and Entourage X: Hold down the Option key and re-launch
the application. In a few seconds, you'll be greeted with a dialogue box
asking if you want the "Typical Rebuild" or the "Advanced Rebuild." Choose
"Typical."
b.. Entourage 2004: Hold down the Option key and re-launch the
application.
In a few seconds, you'll be greeted with the Entourage 2004 database
utility
c.. Select Rebuilt database and then try opening your emails