It usually requires a complete replacement of the keyboard since individual keys cannot be replaced. Well... except in expensive mechanical keyboards, but no laptops have mechanical keyboards.http://www.parts-people.com/index.php?action=item&id=5070
Laptops and the vast majority of desktop keyboards use membrane keyboards. Very inexpensive to manufacture compared to mechanical keyboards, but they are basically an integrated single unit. Once one key goes bad the complete keyboard must be replaced.