Microsoft SQL Server Agent Create Job Error
When you try to create a Microsoft SQL Server Agent Job in Microsoft SQL Server 2005 Management Studio, you get the error:

Unable to cast object of type ‘Microsoft.SqlServer.Management.Smo.SimpleObjectKey’ to type ‘Microsoft.SqlServer.Management.Smo.Agent.JobObjectKey’. (Microsoft.SqlServer.Smo)

Installing Microsoft SQL Server 2005 SP2 solved the problem