Private Const ERROR_ALREADY_EXISTS As Long = 183&
 
Private Type SECURITY_ATTRIBUTES
    nLength As Long
    lpSecurityDescriptor As Long
    bInheritHandle As Long
End Type
 
 
Private Declare Function CreateMutex Lib "kernel32.dll" Alias "CreateMutexA" _
   (ByRef lpMutexAttributes As SECURITY_ATTRIBUTES, _
    ByVal bInitialOwner As Long, _
    ByVal lpName As String) As Long
 
 
Public Declare Function CloseHandle Lib "kernel32.dll" _
   (ByVal hObject As Long) As Long
 
Public hMutex As Long
 
Private Sub Main()
    Dim hSec As SECURITY_ATTRIBUTES
 
    hSec.nLength = Len(hSec)
    hMutex = CreateMutex(hSec, False, "FINGERWEG")
    If Err.LastDllError = ERROR_ALREADY_EXISTS Then
        MsgBox "Finger weg von meiner Exe, du Böser du!"
        Exit Sub
    End If
 
    Form1.Show
 
    ' W I C H T I G !!!!
    ' Später vielleicht in der Form1 Terminate Event
    ' CloseHandle hMutex .... sonst kann man die eigene Exe nicht
    ' mehr starten und muss rebooten.
End Sub