From the description your best bet would be to find someone to help you get it out of the window or it will eat your lunch on leakage. I think there are kits that would go around it on the outside and seal but if that is possible than removing it should be also which is by far the best option. A window sealing kit on the inside would help but holes you can see light through...I hope you have milder winters than we do.

When we bought our house, it had a built in wall unit. That is basically a huge window unit that went into a hole cut in the wall instead of a window.

For winter (in South Dakota) the thing was wrapped in insulation and then heavy plastic, and then a wooden box was put over all that and attached to the house so it would stay.

On the inside, we got good quality window plastic wrap, and taped along the frame and effectivley shrinkwrapped the opening into the house. Then since the frame was bad and cold air was coming in between the frame and the wall, we ended up putting another layer and extending right out onto the wall. That helps a lot - when you have windchills around -40, you really want to stop that cold air!

When we remodeled we went to window units. So much easier to take the unit out of the window and simply close it!