Set the backcolor and enabled properties according to the value of the checkbox e.g.

Private Sub CheckBox1_Change()
    With Me
        If .CheckBox1.Value = True Then
            .TextBox1.Enabled = True
            .TextBox1.BackColor = &H80000005
        Else
            .TextBox1.Enabled = False
            .TextBox1.BackColor = &HE0E0E0
        End If
    End With
End Sub
Set the initial value of the textbox to the false values.