'Add two buttons to your page, call them btnSignIn and btnRegister.
'Assuming you are doing this on Page_Load, use a session variable to hold someone's login status:

Sub Page_Load
  If Session("signedIn") = "True" then
    btnSignIn.Visible = false
    btnRegister.Visible = false
    'hyperlinkRegister.Visible = false
    HyperLink1.Visible = true
    'do whatever you need to here for protected page.
  else
    btnSignIn.Visible = true
    'If you are partial to the button, use the function below. If not, use a hyperlink.
    'hyperlinkRegister.Visible = true
    'hyperlinkRegister.Text = "Register"
    'hyperlinkRegister.NavigateUrl = "register.aspx"
    btnRegister.Visible = true
    'Still don't know what hyperlink1 is for :)
    HyperLink1.Visible = false
    'do whatever you need to here to protect the page.
  end if
End Sub

Sub btnSignIn_Click(S As Object, E As EventArgs) Handles btnSignIn.Click
  If txtUserName.Text = "geeta" And txtPassword.Text = "geetajlo" Then
    Session("signedIn") = "True"
    MsgBox("Welcome geetajlo")
    btnSignIn.Visible = False
    btnRegister.Visible = False
    HyperLink1.Visible = True
  Else
    MsgBox("Wrong UserName or Password. Please Try Again.")
  End If
End Sub

Sub btnSignOut_Click(S As Object, E As EventArgs) Handles btnSingOut.Click
  Session("signedIn") = ""
  'If you have a page below or if you need redirection..
  response.redirect ("logout.aspx?logout=successful")
End Sub

Sub btnRegister_Click(S As Object, E As EventArgs) Handles btnRegister.Click
  response.redirect = "register.aspx"
End Sub