Just like TonkaToys mentioned: try "Frontline - Spot On" for cats; even though it sounds like some FPS title, it worked wonderfully well for my cat when she was infested with fleas about 3 years ago. It's produced by a company called "Merial".
The cat just needs a few treatments (one every X weeks, depending on when the 'remaining' fleas come back; every time exponentially less though), and after 3 or 4 treatments: bye bye fleas, and never come back.