A SELECT statement is a query.  You don't use ExecuteNonQuery when you are,
in fact, querying the database.  Since your query *can* return results, you
need to use the ExecuteReader method of your command, which returns a
DataReader object that you then use to look at the query results.

Try this:

Try
   Dim con As New OleDbConnection("Provider=MicroSoft.Jet.OLEDB.4.0; Data
Source = " & Server.MapPath("Someplace.mdb"))
   Dim cmd As New OleDbCommand("SELECT * from Cus where EMailAddress =" &
QM & Trim(txtEMail.Text) & QM, cnString)
   con.Open()

   Dim dr As Oledb.OledbDataReader = cmd.ExecuteReader()

   If dr.HasRows Then
      txtFirstName.Text = "Yes"
   Else
      txtFirstName.Text = "No " & Now
   End If

Catch ex As Oledb.OledbException
   'Database exceptions handled here
Catch ex As Exception
   'All other exceptions handled here
Finally
   dr.Close()
   con.Close()
   con.Dispose()
End Try

The method doesn't return a boolean at all, it returns an integer. If
you would have use Option Strict the compiler would have told you that.

The return value is the number of affected records. As a select query
never changes any records, your call will always return zero.

You should always use SQL Parameters for your queries. Otherwise you can be
the victim of a SQL injection attack.