Unlock the window. If the locks are painted shut, cut as much of the paint off as you can with a utility knife. Tapping rotary locks lightly with a hammer usually loosens them. If the window has pins, wedge a flat-head screwdriver under the head of each pin and tap the screwdriver with a hammer to loosen it.
Cut the paint seal between the frame and the casing on both sides of the window with a utility knife.
Wedge a stiff putty knife between the bottom of the window and the sill and move it from one side of the window to the other. You may need to tap the knife gently with a hammer to get it into the gap.
Work a flat bar into the gap you created with the putty knife and pry the window up. Start prying at one side of the window, and as soon as it moves, pry the other side by the same amount.
Repeat this procedure from outside the house if necessary. If a ladder is required, position it carefully and anchor it to the house before climbing.
Lift the window open from inside after you've opened it about 2 inches with the pry bar. If you still need to pry it, use a long piece of two-by-four lumber as a lever.
Rub a bar of soap along the window tracks when you get the window open, then slide the window up and down a few times until it gets easier to operate.