Forms :: Change Value In Textbox - Capture User Login

I would like to put a text box (user) and checkbox (check100) on a form, that when the checkbox is not checked the value in Gender switches. In an AfterUpdate I would use a code to capture the user login.

Code:
Private Sub Check100_AfterUpdate()
Me.User = Environ("UserName")
If Me.check100 = 0 AND Me.Gender.Value = "Male" THEN
Me.Gender.Value = "Female"
ElseIf Me.check100 = 0 AND Me.Gender.Value = "Female" Then
Me.Gender.Value = "male"
End If
End Sub