Sounds suspiciously like a stuck key.
I am presuming this is a desktop PC. Turn the PC off and unplug the keyboard. Turn the keyboard upside down and give it a shake; you may well see dust/muck fall out.
Plug the keyboard back in and then boot the PC. Do not touch any keys on the keyboard. Select the on-screen keyboard (Start/All Programs/Accessories/Ease of Access/On-Screen Keyboard) and try typing with this - it should type without the problem. Try the keyboard again and see if it works correctly; if it doesn't you will need to replace it.