I booted XP today using BootCamp (not the first time) and everything ran fine. When I was done with Windows, I shut down and tried to boot back into OSX.

When I booted OSX, the startup gong sounded, the gray apple screen showed, then the light-blue screen came up.
The blue screen came up first with the black loading icon and then the loading icon stopped and the cursor came up over the light-blue screen. Then it went back to loading. It alternated like this for 15 minutes. I didn't install anything on my last OSX boot.

XP still boots fine, but OSX won't boot. There's nothing in the disc drive, it is powered okay, and I zapped the PRAM.

What can I do? PLEASE HELP!