For example, when I type "several years," what appears on the screen is "o.k.pan f.apo".

I don't understand why or when this occurs, except that it may occur inconsistently after I've been writing something for longer than a few minutes.

Usually, I have to save my work and reboot to continue working; however, it seems that sometimes all I have to do is leave the program (Word or Outlook) and go into some other program for a minute and when I return I can continue my work for a while.

Does anyone know why or how to stop this from happening. My O.S. is XP Pro.

Sounds like your keyboard layout is changing to another language for some reason.

Do you have multiple layouts installed? If you do & don't need the other remove it - go to Control panel -> keyboard -> input locales and remove any languages not required for your system.