This can certainly be frustrating. In Word 2003 (which I haven't used in a long time), I believe you will see in the bar along the very bottom of the document window the word "Overtype." If you click, or double-click, or use the right mouse button and click, either it will change to "Insert" or you'll get this possibility in a pop-up menu.

If you don't see such a bar at the document window bottom, it;s the Status Bar that you need to turn on, and this will be either on the View menu or under Tools/Options.

Hope this helps.

Ulodesk