Gary,
> Is there a right or wrong placement? <
It seems to me that almost anywhere that's convenient is fine, so long as it's out of the way of the direct line of fire from your speakers. You should avoid putting it against the wall in the path of a first reflection, assuming you have acoustic treatment there, to maintain symmetry between the left and right sides. I can't imagine what harm would come from putting it anywhere in the rear of the room, but I am confused about your comment that your speakers are there. Are these rear speakers in a surround setup? Either way, if it's against the rear wall behind you I see no problem.
> Which acoustic products work best with windows. <
The goal is symmetric surfaces to the left and right of you for all wall areas between you and the speakers. So if you're six feet in front of the speakers, then the six feet of wall between you and the speakers should be the same on both sides. For best results you also want to avoid reflections in that zone, as explained in the "RFZ" sidebar in my Acoustics FAQ. So if you have treatment on one side and a window on the other, you want some sort of curtain - as heavy as possible - in front of the window.
--Ethan
