Resolved How to rectify a keyboard malfunction.

You have probably hit the "insert" key by accident. Hit it again and you should be back to normal. Hitting the "insert" key should, when you type the next letter, replace the letter to the right of the cursor with the one you type in. It keeps doing that until you hit the "insert" key again. However, when I test it on my Windows 7 system ... ... hitting the "insert" key makes no difference (probably a keyboard malfuntion) but you find that key and hit it anyway.

 

You're welcome ... ... but did it provide the solution?

A quick googling revealed that the "insert" function is disabled in Windows 7 (I don't know about later versions of Windows but probably the same) and has to be enabled in Word (but not possible in Notepad):

In Word go to file, Word options advanced and choose "Use the Insert key to control overtype mode then check the box use overtype mode.

So, no keyboard malfunction, only ignorance on my part. (It seems like I never use "insert".)