Private Declare Function CreateMutex Lib "kernel32" _
    Alias "CreateMutexA" (lpMutexAttributes As Any, ByVal bInitialOwner As _
    Long, ByVal lpName As String) As Long
Private Declare Function ReleaseMutex Lib "kernel32" _
    (ByVal hMutex As Long) As Long
Private Declare Function CloseHandle Lib "kernel32" _
    (ByVal hObject As Long) As Long
Const ERROR_ALREADY_EXISTS = 183&
Private hMutex As Long
Public PreviousInstance As Boolean
 
Public Sub Main()
    'Try to create a new Mutex
    hMutex = 0
    hMutex = CreateMutex(ByVal 0&, 1, App.EXEName)
    'Did the mutex already exist?
    If (Err.LastDllError = ERROR_ALREADY_EXISTS) Then
        PreviousInstance = True
    End If
 
... 
 
    If not PreviousInstance then
        'do update, if no other instance is running
        Call UpdateCheck    'for example
    Endif
    MDIStart.Show
End Sub
 
Public Sub MainEnd()
    If PreviousInstance Then
        PreviousInstance = False
        'Clean up
        ReleaseMutex hMutex
        CloseHandle hMutex
    End If
    'end application
    End
End Sub