Hi Lizabef
It's almost too tempting not to say that Windows slows everything down, but there - I resisted manfully and didn't say it!
Seriously though, the thing that slows down wireless network links is signal degradation, either as a result of increasing distance between the devices which reduces the basic signal strength or because of multi-path reflections where the signal bounces around of objects in the vicinity and the resulting signal that appears at the reciever is 'smeared' for want of a better description. These effects generally work together, with the result that signal quality falls off rapidly as you begin to approach the limits of the usable range of the link. Wireless kit automaticaly reduces the link speed in response to poor signal quality (in an effort to keep the link working), which is why you get the slowdown.
As to your specific question about windows, anything that sits in the signal path between the devices is going to have some effect. However I would have thought that a window, which is basically a big hole in the wall with a thin sheet of glass in it, would have much less effect than say dense brick, structural steelwork inside concrete or even that foil-lined wallboard often used in interior partitions. As long as you are comfortably within the usable range of your devices, a window should present no problem.
(Written using a wireless link coming through a window with metal security bars, across an open area, through a metal fence then through another window, about thirty feet at 802.11b)
