Late 2008 Macbook aluminum won't start up correctly.

So... I don't know what it is, but my macbook won't boot into OSX. It boots into my windows partition fine, but not OSX. When I try and boot into mac, everything is normal at first. You hear the startup noise, and the apple appears. But underneath the apple is a bar that would normally appear if you were updating something. But I haven't installed any updates recently. So I wait a minute, and the update bar doesn't go up at all. After a minute it just disappears. Then there's just the apple like there would be on a normal start. But from here it doesn't do anything. I've waited fifteen minutes and it still won't do anything from there. Anybody got an idea?

Specs: macbook aluminum unibody, 2.4 Ghz intel core 2 duo, 6 gb ddr3 so dimm, OSX 10.6.4