Edit text: insert vs replace typed characters

Anyone out there expert in using Microsoft Word 2003? I can't figure out how to control manual entry of corrections into existing text. I'm not an expert typist by ANY standard, actually a being 2 index finger typist, only occasionally involving a second finger on either hand. My problem is that I'll be typing along and look up to the computer screen to see I've made a spelling, grammar or wording selection error. When I go back into the text to type in a correction I often find that any character I type REPLACES the following character rather than inserts the desired character into the text. I never have ANY idea of what key error might have caused the switch from insert to replace function of keying in characters. I end up having to verify the "uncheck" of replace in the default edit options window, select all text, copy and close and reopen "Word" and paste to restart the document completion process. Anyone know how to avoid this pain in the ass?