Sometimes when kb's are too far away from their receiver it can do that also. Make sure your kb and receiver are adequate distances from each other. Do you have any other wireless devices? What are they... Do you have another computer in the room with wireless items? I would also try a driver update... just to be safe.
Try another kb if you have one laying around.. see if it still happens.. I've had viruses that played with my settings like that. Scan your system.
If none of those things work for you I would suggest getting a new kb...the keypads inside go bad and hit all the wrong things when they do. Not saying that's your only option but definitely one you need to consider.