Updates to system catalogs are not allowed - a big subject and nothing but opening door to trouble by allowing direct updates or modifications to system tables in SQL Server 2005. If I remember correctly since the days of SQL 6.5 version Microsoft has
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Although the guest user remains in all of the user and system databases performing the same general functionality in the SQL Server 2000 and previous versions, at the same time you can revoke the access as mentioned in BOL http://msdn2.microsoft.com/en-US/library/ms177573.aspx
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