I've got this same problem, however, due to a hardening of the SQL Server, I require the user to be part of the Guests group.  Everything (including SQLServer) is running 100% as is, it is only the SQLServerAgent that will not run unless it is removed from the Guests group.  Adding this user to the Users group results in the SQLServerAgent starting successfully!  I'm guessing there is a permissions thing somewhere, however i've been through the system/registry/security policies and can't seem to find where it would be.  Any help would be greatly appreciated!