Code:
Public Class Form1
    Const LWA_COLORKEY As Int32 = 1
    Const LWA_ALPHA As Int32 = 2
    Const GWL_EXSTYLE As Int32 = (-20)
    Const WS_EX_LAYERED As Int32 = 524288
    Private Declare Function apiGetWindowLong Lib "user32" Alias "GetWindowLongA" (ByVal hWnd As Int32, ByVal nIndex As Int32) As Int32
    Private Declare Function apiSetWindowLong Lib "user32" Alias "SetWindowLongA" (ByVal hWnd As Int32, ByVal nIndex As Int32, ByVal dwNewLong As Int32) As Int32
    Private Declare Function apiSetLayeredWindowAttributes Lib "user32" Alias "SetLayeredWindowAttributes" (ByVal hWnd As Int32, ByVal crKey As Int32, ByVal bAlpha As Byte, ByVal dwFlags As Int32) As Int32
    Dim nhwnd As Int32

    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        Dim p As New System.Diagnostics.Process
        p = Process.Start("notepad")
        p.WaitForInputIdle()
        nhwnd = p.MainWindowHandle.ToInt32
        TrackBar1.Maximum = 255
        TrackBar1.Value = 100
    End Sub

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        Transparency(nhwnd, 100) 'Set transparency to 100, or anywhere between 0-255.  Use a trackbar if you want.
    End Sub

    Private Sub Transparency(ByVal hwnd As Int32, ByVal sOpacity As Int32)
        Dim Ret As Int32 = apiGetWindowLong(hwnd, GWL_EXSTYLE)  'Set the window style to 'Layered'
        Ret = Ret Or WS_EX_LAYERED
        apiSetWindowLong(hwnd, GWL_EXSTYLE, Ret)
        apiSetLayeredWindowAttributes(hwnd, 0, sOpacity, LWA_ALPHA) 'Set the opacity of the layered window
    End Sub

    Private Sub TrackBar1_Scroll(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles TrackBar1.Scroll
        Transparency(nhwnd, TrackBar1.Value)
    End Sub
End Class
first of all im sorry for bringing up an old topic
im new at vb and rly dont know how to do this with an already started application
could somebody rewrite it to make it work with and already started application?