iMac stuck on white screen, gear stops spinning

My 2008 24" iMac has stopped booting up all the way. When I hit the power button I do get the chimes that play, and I get to the white screen with the grey apple, however after a few seconds the gear stops spinning.

Here's what I've tried so far:

1. Holding down Shift while booting: get progress bar, but gear eventually stops booting

2. Holding down Option: I get Mac HD, Recovery HD, and Install Disk. Choosing any of them gets me back to white screen, and gear stops spinning.

3. Holding down Command + Option + P + R: Causes reboot, boot after that gets me to white screen, gear stops spinning.

4. Holding down C to boot directly to install disk: Same result, gear stops spinning after awhile.

5. Leaving iMac unplugged for a few hours. Same result.

I'm pretty desperate at this point. I'm looking for any ideas.