Macbook shows loading bar then shuts down

Every time I start my Macbook, a loading bar appears and automatically shuts down after the bar is about 1/4 full. I have Windows 7 installed using Bootcamp and I can access all the files in the Mac OS partition, so I'm assuming it's something wrong with the system files. Neither of my USB ports are working, so I'm unable to back up the Mac OS partition and reinstall it. I've tried booting using CMD+R, but repair disk doesn't work.

My question is, is it at all possible to reinstall Mac OS without losing any of my files?