If you're having trouble opening your windows (the old sash drawn style), try giving it a whack first as if you're trying to close it more. Sometimes the paint sticks, and the whack breaks the seal between the window and its frame.
The first time I saw my dad do this I was disgusted by how easily he opened the window after I'd been pushing on it with all my might. Whack, swush, voila! But since it works as well for me now, I admire the effectiveness of brain over brawn.