Unable to edit Calendar appointments or Contacts in Outlook 2007

I have the following environment:
Micrsoft SBS 2003 Server SP1 with Exchange 2003 SP2
40 Outook 2003 Clients
5 Outlook 2007 Clients
All client PCs running Trend AV
All Clients fully patched via WSUS (Office and Windows)

The problem is as follows
Any Client PC that has Outlook 2007 installed cannot edit, change or dismiss reminders from the calendar. The same goes for Contacts.
When the idem is created in the calendar there are now error messages to indicate that there have been any problems

When someone tries to open the appointment by double clicking it, the error (Cannot open item. Unknown error) appears
When trying to dismiss the reminder that is generated a similar error is displayed

If the appointment is created by someone who is using Outlook 2003 then the appointment can be edited by anyone, including the users on Outlook 2007
If the appointment is created by someone who is using Oulook 2007 then the appointment cannot be edited by anyone.

I have tried the following
1. Re-creating the mailbox on the exchange server
2. Disabling the Antivirus
3. Removing Office 2007 completely (Also deleting the resitry entries) and re-installing from the media

There are no error messages in the even logs on the server, even with all the Exchange logging turned up to maximum
There are also no errors in the event log on the client PCs

Any help would be greatly appreciated
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