SQL Server Books On Line (BOL) recommends that you disable the guest user in every Database as a best practice for securing the database server. Microsoft on 28th April 2011 release KB article 2539091 outlining a change to this recommendation.
This KB article suggests that you DO NOT disable this user due to some Microsoft SQL Server features will not work. The error message that you receive may vary slightly but the underlying cause is insufficient privileges in the MSDB database.
For further information on the 3 symptoms identified by Microsoft read the KB article 2539091 to ensure this relates to the issue that you are experiencing. The end result is to Grant Connect permissions to Guest in the MSDB database.
I hope this helps all of you.