Unable to boot back into OS X

I have installed Win XP Pro successfully on my Macbook Pro. It works without glitches thus far. However, I am now unable to boot back into OS X. I can get to the select screen on bootup and choose my mac volume. Usual Mac startup proceeds but then hangs forever. After a forced reboot it defaults back to Windows XP.

Macbook Pro 2.16
2 GB RAM

I installed on the C: drive per the instructions. Never did the onmac hack, was using boot camp.

Okay, ran disk first aid.

My mac volume now has an invalid volume header.
Also has an invalid extent entry.

There is a thread about this on the apple support forums:
here