I second what Antares said.  I see the error because there are two or more rows that have the info that you are deleting.  So if you query a table and get the customer firstname, lastname and if you want to delete John Smith, chances are that there are many John Smiths in the table.  Which one does Sql Server need to delete?  You need to pull the primary key as well so that Sql Server knows what row it should delete.