First of all, Try connecting another keyboard, if this does not work then
Re-install the language you are using, or look out for settings on Control Panel.
This could be a font problem, or either someone set (can be done by virus too) Special character on those keys, resulting in typing a completely false letter than the one you are trying.
Last, try booting into Safe Mode (Spam F8 upon powering up the computer), and see if it happens in Safe Mode too. If it does, you have problematic settings, if it doesn't then it is caused by a virus.