Gently does it...
Without being able to see any pics....from my experience, if you lubricate the the top rail of the window and the grooved profile it fits into, liberally, with something like WD40, then open the window as it would normally, swinging the bottom out ,and with all hardware removed, only taking it as far up and out as it will go ( another set of hands will make this job infinitely easier), taking the weight of it, as it were, with the window open approximately 90 degrees to the body. Then, with a rubber mallet, gently tap the window sideways, preferably with a small block of wood nestled into the side of the window, near the top so it won't bounce or bend the frame, to blind the blows from the rubber mallet.You will find there will be a " sweet spot ", somewhere if you raise or lower the window frame as you tap sideways, and it should slide out. There may be a little resistance but take your time and do not force things, you should be right. There should also be a tiny nylon/teflon block that is housed in a small slot in the top of the window rail, that the small centering screw ( which it sounds like you have already removed ), is screwed into. Watch out for it as you draw the window out, as it is just sitting there. Hope this makes sense and is not too long winded. Good luck.