It could be due to some harmless virus playing pranks. Or did you drop any liquid on the keyboard?
If that helps you:
*www.google.com/search?hl=en&client...l1i&q=keyboard+types+wrong+letter&btnG=Search
Perhaps bit advanced!
*www.google.com/search?hl=en&client...t=result&cd=1&q=remap+entire+keyboard&spell=1