It may be a software fault - you can try uninstalling and reinstalling the keyboard driver or doing a factory reset to make sure its not another application causing the problem. If this fails it is probably a hardware fault (e.g. the keyboard itself) You could try keyboard cleaning products / switch cleaner. If that doesn't work then the keyboard will need replaced and if a new keyboard doesn't fix it it may be a motherboard problem - and a new motherboard would properly cost more than the computer is worth.