Ever seen an Outlook installation that's slow to close because of huge open .PST files? Those cases where you close Outlook but the tray icon turns gray and remains there for up to 20 minutes while the Outlook process continues writing bytes, or those annoying 'Outlook process' messages when you close Outlook just before shutting down?

I've found these registry entries fix the problem in all of the dozens of times I've used them. Be logged on as the user, and wait for the Outlook process to completely finished first. Obviously, as with all registry changes, back it up first.

For Outlook 2007:

Windows Registry Editor Version 5.00

[HKEY_CURRENT_USER\Software\Microsoft\Office\12.0\Outlook\PST]
"PSTNullFreeOnClose"=dword:00000000

For Outlook 2010:

Windows Registry Editor Version 5.00

[HKEY_CURRENT_USER\Software\Microsoft\Office\14.0\Outlook\pst]
"pstnullfreeonclose"=dword:00000000