Presupun doar, ca nu ati configurat bine setarile de securitate. 

O alta posibila cauza este specificata de acest articol: http://support.microsoft.com/kb/314530.

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Cristian Andrei Lefter, SQL Server MVP
MCT, MCSA, MCDBA, MCAD, MCSD .NET,
MCTS, MCITP - Database Administrator SQL Server 2005
http://sqlserver.ro