DesktopDisplay.AvailableTop tells you where the top most part of the available area on the screen is. This should do the trick:
var oBounds as Rect = self.Bounds
oBounds.Top = DesktopDisplay.DisplayAt(0).AvailableTop
self.Bounds = oBounds
(remember that Window.Top is the window’s content area top)
Edit: I forgot that you can’t manipulate Window.Bounds directly.