Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As _
  System.EventArgs) Handles Button1.Click
  ...
  PictureBox1.Image = CaptureWindow(hwnd)
End Sub

Public Function CaptureWindow(ByVal hwnd As Integer) As Bitmap
        Dim hdcSrc As Integer
        Dim Breite As Integer
        Dim Hoehe As Integer
        Dim hdcDest As Integer
        Dim hBm As Integer
        Dim hOldBm As Integer
        Dim BmNeu As Bitmap
        Dim RECT1 As RECT
 
        'hDC des Quellfensters
        hdcSrc = GetDC(hwnd)
 
        'Größe ermitteln
        GetWindowRect(hwnd, RECT1)
        Breite = RECT1.right - RECT1.left
        Hoehe = RECT1.bottom - RECT1.top
 
        'hDC des Zielfensters
        hdcDest = CreateCompatibleDC(hdcSrc)
 
        'Neue Bitmap anlegen
        hBm = CreateCompatibleBitmap(hdcSrc, Breite, Hoehe)
 
        'Bitmap selektieren
        hOldBm = SelectObject(hdcDest, hBm)
 
        'Quellfenster in neue Bitmap kopieren
        BitBlt(hdcDest, 0, 0, Breite, Hoehe, hdcSrc, 0, 0, SRCCOPY)
 
        'Auswahl wiederherstellen
        SelectObject(hdcDest, hOldBm)
 
        'Aufräumen
        DeleteDC(hdcDest)
        ReleaseDC(hwnd, hdcSrc)
 
        'In GDI+ Bitmap umwandeln
        BmNeu = Image.FromHbitmap(hBm)
 
        'Bitmap freigeben
        DeleteObject(hBm)
 
        Return BmNeu
End Function