Coma en calculadora

Supongo q si programas en fox programaras en visual basic, espero q me ayudes, es muy urgente. Mira debo ser idiota porq no se como hacer q en mi calculadora pueda calcular con decimales, no se introducir la coma sin q se vaya el número anterior,mira te pongo el código oki?
Option Explicit
Dim T1 As String
Dim Op As String
Dim F1 As Double
Dim res As Double
Private Sub cambio_Click(Index As Integer)
txt1 = txt1 * -1
End Sub
Private Sub Cmdc_Click()
txt1.Text = ""
End Sub
Private Sub cmdce_Click()
txt1.Text = "0"
T1 = ""
F1 = ""
Op = ""
End Sub
Private Sub Cmdigual_Click()
If Op <> "" Then
If Op = 0 Then
res = Val(F1) + Val(T1)
ElseIf Op = 1 Then
res = Val(F1) - Val(T1)
ElseIf Op = 2 Then
res = Val(F1) * Val(T1)
ElseIf Op = 4 Then
If T1 = 0 Then
res = "E"
Else
res = Val(F1) / Val(T1)
End If
End If
txt1.Text = res
T1 = ""
F1 = res
Op = ""
End If
End Sub
Private Sub cmdoff_Click(Index As Integer)
Unload Me
End Sub
Private Sub com_Click(Index As Integer)
txt1.Text = ","
End Sub
Private Sub num_Click(Index As Integer)
T1 = T1 & Index
txt1.Text = T1
End Sub
Private Sub oper_Click(Index As Integer)
F1 = txt1.Text
T1 = ""
Op = Index
End Sub
Private Sub txt1_Change()
txt1.Locked = True
End Sub
Gracias