help …stop cursor from jumping back to a different line while typing

I have Windows XP on my laptop. While typing in any word processing program or email program, the cursor will “jump” back to an earlier line while I am typing and begin to enter the letters in the middle of something I had previously typed. Since I do not always look at the screen while I am typing, I get very frustrated to find that, when I do look up, I have to figure out what I have to cut and paste back to the end of the document…it is a great time-wasting and frustrating “glitch”! Any ideas on what is causing this and how to fix it? I am assuming it is a Windows XP problem because it happens it several different programs (even when I was typing this note!!!)