Window stopper preventing operation of window

This part is known as a vent lock. Normally, you just press it in and slide it to one side to lock it in place. Since yours sounds broken, you will need to remove it.

The trick is, vent locks are not meant to be replaced. It's frustrating, but that's just how they're designed. You will need to break it out to replace it. Take a thin putty knife, slip it under the outside flange, then force up to break it out. To prevent damaging the wood finish, tape a piece of thin cardboard in place before you force it out. That should work as a cushion of sorts.