Visual Basic - ventana activa sistema operativo

como puedo hacer que mi aplicacion sea la ventana activa a travez de codigo

Si quieres que tu ventana esté activa por encima de las demás, en http://support.microsoft.com/support/kb/articles/q184/2/97.asp tienes la forma de hacerlo, se hace con:

Public Const SWP_NOMOVE = 2
Public Const SWP_NOSIZE = 1
Public Const FLAGS = SWP_NOMOVE Or SWP_NOSIZE
Public Const HWND_TOPMOST = -1
Public Const HWND_NOTOPMOST = -2
Declare Function SetWindowPos Lib "user32" Alias "SetWindowPos" _
(ByVal hwnd As Long, _
ByVal hWndInsertAfter As Long, _
ByVal x As Long, _
ByVal y As Long, _
ByVal cx As Long, _
ByVal cy As Long, _
ByVal wFlags As Long ) As Long
Public Function SetTopMostWindow(hwnd As Long, Topmost As Boolean) _
As Long
If Topmost = True Then 'Make the window topmost
SetTopMostWindow = SetWindowPos(hwnd, HWND_TOPMOST, 0, 0, 0, _
0, FLAGS)
Else
SetTopMostWindow = SetWindowPos(hwnd, HWND_NOTOPMOST, 0, 0, _
0, 0,FLAGS)
SetTopMostWindow = False
End If
End Function