unable to upgrade exchange 2007 SP1 to SP2

error message:

exchange 2007 sp2 setup previously failed while performing the action install.you cannot resume setup by performing the action buildtobuild upgrade

Resolution:

Go through the following registry keys and delete all the "Watermark" and "Action" items. The install will proceed successfully.

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Exchange\v8.0\AdminTools
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