Get a can of air, and blow between the keys. Lost of junk can fall between the keys, and cause problems. Hold the can right between the keys. I've saved my laptop when my nephew was eating crackers using the computer. I couldn't beleive what came out of it!
If it is truely broken, you can replace the keyboard, but it isn't simple.
-David
2006 Microsoft Most Valueable Professional (MVP)
2006 Dell Certified System Professional (CSP)