osx 10.5.8 not starting up

After the first battery run-flat in a couple of years on my 1st gen MacBook, it refused to restart, halting at the first grey screen with the spinning wheel going round for over 15 mins. After some restarts, i tried an alt restart, as i have bootcamp and windows xp installed on a partition. Windows xp opened first time. Tried a reinstall on an external drive as my internal drive is a bit tetchy, which seemed to work until restarting after downloading software updates. I am reluctantly writing this on my mac in windows xp, please help if you can! ;D

You haven't said anything about repairing your hard drive yet?
Resolve startup issues and perform disk maintenance with Disk Utility and fsck