..the "Insert" key. For "Word", this toggols "Overtype"/"Insert" for corrections. you must have pressed the Insert key accidentally. Look at the ststus bar at the bottom for "OVR". This is also an option whichn ou can change at tools>Options
I was typing today and noticed that (for no reason I can work out) now when I select text and try to type over it, or press backspace, instead of replacing the text it just inserts the new text in front.
This just happened when I was working on a long document (50 pages), but effects all documents. I didn't change any settings.
This is Office XP on a new Windows MCE-running HP notebook, but one of the old computers at work has had the same problem for a long time too.