It sounds like your hard drive is failing. I'd go to Apple ASAP.
I've had a hard drive fail on me on this macbook pro because I kept dropping the computer repeatedly but they just exchanged a new one for me.
I find if you're fresh out of warranty, they're really good about not charging you or at least charging you only for the part. But since I abuse my computers like you wouldn't believe, I get applecare and they replace everything for me within 30 min... Even when I broke it by dropping it.
I love Apple's customer service at any rate. They're always really helpful.