Thank you for your quick answer.
Well, I've finally suceeded in writting my own save/restore window position software. I'm using the SimConnect SDK to add a menu in FSX to drive this "save/restore" stuff.
It does now a nice job, except that it works only when FS is in windowed mode.
The similar softwares I saw on the web is called "Panel_position" : http://forums1.avsim.net/lofiversion/in63144.html
I still have a problem : from time to time, when switched to full screen FS has the very bad idea to change the display arrangments, and also to crash miserably :-(
For info, my config has 6 screens driven by 3 radeon 5850
BTW, are you aware of a software allowing to precisely match view in adjacents screens ? (the main view is made with 3 monitors in eyefinity mode, with another 2 side monitors)
Thanks,
Frédéric.