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You probably should read the tutorial section, there're a couple of tutorials for window scenes in there... you can find the link to mine in my signature :D
I'm talking about the game window, like when you play the game. Basiclly the game screen. From what I understand, your tutorial is about in-game windows, such as those on the menu
Ah, alright... Well, I've seen a way to do this, but it's been ages ago... but it's definately possible, though I wouldn't reccomend it for several reasons...
And sorry for my misinterpretation...
240 x 160? Are you planning on making a gameboy-styled game? The graphics in RTP are FAR too big for that!
Plus as far as I know there's no "Game.exe" to .gba.