|Have you tried both PS/2 and USB keyboards?|
It's unlikely but possible a PS/2 port's circuits could be damaged (PS/2 keyboards and mice (and ports) are not hot pluggable - sometimes people plug in or unplug them when the computer is running) .
If you have Word or Word reader installed, try un-installing it in Add/Remove Programs, and installing it again.
Try searching with: keyboard wrong character
(without quotes) - there are lots of "hits" for that