After uninstalling Symantec Anti-virus I found that when I started Outlook 2007 it came up with the following error:

The Add-in “Symantec AntiVirus 10.1″ (c:\program files(x86)\Symantec Antivirus\vpnsece4.dll) cannot be loaded by Outlook. Please contact the Add-in manufacturer for an update. If no update is available please uninstall the Add-in.

The problem was that the addin didn’t appear in Tool->Trust Center->Add-ins so I couldn’t uninstall it from there.

Fortunately, deleting the file extend.dat from c:\users\<User Name>\AppData\Local\Microsoft\Outlook and restarting Outlook done the trick. That was obvious – NOT!!

NOTE: I am running Vista x64 so if your running a different version of Windows the file is likely to be located elsewhere. Possibly C:\Documents and Settings\<User Name>\Local Settings\Application Data\Microsoft\Outlook