I've had this a few times, most likely hardware related and you'll need a new keyboard. Just to make sure though, find another keyboard from somewhere that you know works, and plug it into your computer. If it works, then you can safely say yours is broken and you need a new one.
Alot of dirt can get inbetween the keys and cause multiple keys to be pushed at once, so before you buy a new one, you could try taking the keys off and cleaning the inside of it first then trying it again.