1.Open Start>Run> Type without Quotes: "osk" and press enter.
2.Press those key's which were creating troubles and check those keys on the Osk.
3. If the keys on Osk Works fine then there's a problem with your Keyboard and if the Osk is also facing the same problem then you might have to Format the System because some malwares attack the system drive and alters the behaviour of custom keys.

What this means is, you start the OSK = On Screen Keyboard, click on some letters, maybe the ones that aren't right, then click the Backspace, do this On The Screen with the mouse. If it's working right and the characters are correct, it's the keyboard. If the same errors as the keyboard are showing when you click on the letters on the screen keyboard, it's a bigger system problem.