server installation problem

hi,
i am trying to install siebel gateway server in xp
professional under the user sadmin.and sadmin is under
the goup of administrator and power user. i am getting
the error message"T he user name provided cannot be used
as a service accont.Logon as service required."
i gave the "logon as Service" privilege to the user
sadmin.i am stuck here. please some one help me.....