replace the whole keyboard; they can be found on ebay for about £10. When you've got the new keyboard, you should be able to google the instructions for replaciong the whole sealed unit. It's about a 10 minute job. If you don't feel up to that, just take into into any good pc repair shop (not pc world!) and they will charge you about £10 to do the job for you.
ABCICT, June 2008