Resuming Exchange Installer

Half way through running the SP3 installer from the command line upgrading from Exchange 2007 SP1 to SP3, sod's law kicked in and we had a DNS failure. The installer stopped and as a result I couldn't resume the  upgrade. The installer would error out saying the services were already installed (all sercies that I was running which was Mailbox, Client Access and Hub Transport).

It turned out all instances of the `Watermark` and `Action` entries under the following locations in the registry needed removing:

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Exchange\v8.0\ClientAccessRole
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Exchange\v8.0\HubTransportRole
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Exchange\v8.0\MailboxRole
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Exchange\v8.0\UnifiedMessagingRole
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Exchange\v8.0\ClusteredMailboxServer

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