xpath query that will search for a node with a certain value.

I need to search my xmlDocument for a <loginAction> node such that the value of <username> is equal to a certain value, and <action_name> is equal to a certain value. Here is the routine I need to complete:

    Public Shared Function can_do(ByVal action_name As String) As Boolean
        If xSecurity.DocumentElement.SelectSingleNode("need an xpath statement here") IsNot Nothing Then
            Return True
        Else
            Return False
        End If
    End Function


Here is the xmlDocument:

<SecurityDataset>
  <loginAction>
    <username>mylogin</username>
    <action_name>not-run_funding_reports</action_name>
  </loginAction>
</SecurityDataset>