Software update froze, now can't boot or repair disk

Hi,

Today, in preparation for Mavericks, I started downloading any software updates that were available. I had one update to do (a itunes/mountain lion combo, I believe). So I clicked install update, and was prompted to restart. Immediately the screen changed to 'installing updates'. It got to about 25%, and stayed there with '2 minutes remaining' for about an hour.

After waiting, I decided it was frozen, and restarted the computer. Bad move, because now I can't boot. A normal boot takes me to the Apple logo with a progress bar on the bottom, it sits there for a minute then turns off.

So I tried booting into the recovery utilities. Verify Permissions turns up a ton of incorrect permissions for iTunes. I can't repair them, though, because the button is greyed out. I tried verifying the disk, and it confirms there are errors, but clicking Repair Disk tells me the disk can't be repaired, and that I need to erase the drive. I don't want to do this, is there a way to fix it? Stupidly, I don't have a backup. I know I can manually transfer my most important files from the drive to an external using the command line, and re-install OSX, but I'd rather avoid doing this.