Does the problem only manifest itself on your word processor or does it happen with other programs?
It could be many things.
Which word processor do you use? If MS Word, try downloading "Open Office", which is a free suite of programs that can read/write in Word/excel, PowerPoint etc formats.
If that experiences the same problem, then you may have to look at keyboard/keystroke recognition problems.
PS: Do you have 2 "return" keys? If so, does the problem occur with only one of them or both?
If using a laptop, do you have access to a USB (desktop) keyboard. If a desktop, try another keyboard.