Form Disappear Automatically

[MacArthur]

This is very strange, I have a form, a very simple "Hello World" type of form. If I just do F5, the form shows up, no problem. Now I added a module, with the following code:
***********************
Module MainModule
Dim frm1 As New frmLogin1()

Sub Main()
If PrevInstance() Then
msg = "This Program is already running. Please shut down the currently running"
msg = msg & " process before starting a new one"
MsgBox(msg, vbInformation)
Exit Sub
Else
Startapplication()
End If
End Sub

Function PrevInstance() As Boolean
If UBound(Diagnostics.Process.GetProcessesByName(Diagnostics.Process.GetCurrentProcess.ProcessName)) > 0 Then
Return True
Else
Return False
End If
End Function

Sub Startapplication()
frm1.Show()
End Sub
End Module
**********************************

And I changed the startup object to "sub main" in the project property.

Now, when I do F5, the frmLogin shows, and then disappear automatically...

Can anybody tell me what I did wrong? many thanks...