Causes validation problem with error provider

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VB

In my Login form i have 2 textboxes (txtUsername and txtPassword) and 3 buttons- btnLogin, btnBack and btnCancel. I used errorProvider component to validate the 2 textboxes. Now the problem is, along with the Login button, the back and cancel button is also causing validation though i set the causes validation property of back and cancel button to 'false'. I want only the Login button to cause validation and not all the buttons in the form.
I used the following code to validate the textboxes and i am using visual studio 2010.

Private Sub txtUserName_Validating(ByVal sender As System.Object, ByVal e As System.ComponentModel.CancelEventArgs) Handles txtUserName.Validating
        ErrorProvider1.SetError(txtUserName, "")
        With txtUserName
            If .Text = String.Empty Then
                BtnCancel_LoginForm.CausesValidation = False
                BtnBack_LoginForm.CausesValidation = False
                'cancel the event
                e.Cancel = True
                ErrorProvider1.SetError(txtUserName, "Required field")
                Label2.ForeColor = Color.Red
            Else
                Label2.ForeColor = Color.Maroon
            End If
        End With
    End Sub

Cant detect where and what is the mistake.. please somebody help....