How to stop it typing over your text: try doubleclicking the OVR in the grey bar at the bottom of the Word window, so that it becomes greyed out.
A combo of that and the Insert key should work to stop it from over-typing or over-writing when the Insert key alone doesn't.
I guess occasionally it just happens, or maybe you've accidentally done something to grey out the OVR. It's happened to me quite a few times and I spent ages trying to solve the problem. Hope this helps others.