It sounds like the solenoid on the starter is not engaging. The solenoid kicks a gear out which engages to the engine's flywheel thus spinning the engine to start it. If it doesn't kick the gear out, you will hear the starter spinning but not engaging with the engine.
I would remove the starter, take it to a parts store, and have them bench test it to see if that is what is happening. Most will do it for free, so there is no money out of pocket. If I'm correct, you need to replace the starter.
Let me know if you have other questions.
Joe
Tuesday, May 26th, 2009 AT 1:45 PM