To disable MSN Messenger integration in Microsoft Outlook Express:

Key Name HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Outlook Express
Value Name Hide Messenger
Value Type REG_DWORD
Value Data 0x00000002

And Outlook:

Key Name HKEY_CURRENT_USER\Software\Policies\Microsoft\Office\10.0\Outlook\InstantMessaging
Value Name ForceDisableIM
Value Type REG_DWORD
Value Data 0x00000001