Error 51E = 1310 which should translate to this Error Message:
The group may not be disabled.
Built-in system groups cannot be deleted. The group you have selected to be deleted is a system group.
Not sure how this relates to the service pack installation.
Are all the SQL services enabled (manual or automatic no Disabled)?
Known issues with this security update
The following table lists the known issues with this security update. Visit the Microsoft for resources and tools to keep your PC safe and healthy. If you are having issues with installing the update itself, visit for resources and tools to keep your PC updated with the latest updates.
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Article number Article title 953739 (http://support.microsoft.com/kb/953739/ ) SQL Server 2005 installers do not update an instance of the SQL Server service that is in a disabled state 953741 (http://support.microsoft.com/kb/953741/ ) SQL Server 2005 installers stop dependent services 955250 (http://support.microsoft.com/kb/955250/ ) Error message when you try to uninstall a SQL Server 2005 Express Edition hotfix package: "1731 The source installation package for the product  is out of sync with the client package" 957008 (http://support.microsoft.com/kb/957008/ ) SQL Server 2005 Service Pack 2 is not successfully updated when you try to install or to uninstall a security update