Well if you've already tried restarting your computer and cleaning the keyboard, my best suggestion is to get your keyboard replaced, sounds more like a hardware issue, most likely nothing to do with any software or anything you could probably fix yourself. Just a guess though, good luck!
perchik, Try cleaning your laptop's keyboard with a miniature vaccuum or a special dust cleaner. If all else fails,you could get keyboards for laptops on ebay - i personally changed my keyboard for inspiron 6000.
or, you could pry the key off (requries a little force) and then check to see if anything's stuck there ... if not, then align notches under key with bars / tabs and press down ... try the key by pressing it a couple of times, if it doesn't feel right, something probably didn't snap in all the way, so try again (aka remove, align, press down)