HKEY_LOCAL_MACHINE\Software\Microsoft\Office\11.0\Outlook\DisablePST
For Outlook 2007
HKEY_LOCAL_MACHINE\Software\Microsoft\Office\12.0\Outlook\DisablePST
Some Tech Stuff
For Outlook 2007
HKEY_LOCAL_MACHINE\Software\Microsoft\Office\12.0\Outlook\DisablePST