Public Sub PW(ByVal Text As String, ByVal Box As TextBox)
        Dim rnd As New Random
        Dim Zeichen As String = _
          "abcdefghiklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890"
        Dim i, j, max As Integer
        Dim data As String
        Dim warten As Integer = 1000
        For j = 0 To Text.Length
            Dim TimeOut As Integer = warten + Environment.TickCount()
            Do While Environment.TickCount() <= TimeOut
                Application.DoEvents()
                data = ""
                For i = 1 To Text.Length - j
                    data &= Mid(Zeichen, rnd.Next(1, Zeichen.Length), 1)
                Next i
                Box.Text = Mid(Text, 1, j) & data
                TimeOut = warten + Environment.TickCount()
                Do While Environment.TickCount() <= TimeOut
                    Application.DoEvents()
                Loop
            Loop
        Next j
    End Sub

Call PW("[Passwort]",Textbox1)