macbook update 10.5.7 crash; no difference with safe boot or PRAM

Hi,

I tried to install the mac/security update 10.5.7 last night. I closed all my programs, selected 'restart', the purple colour background appeared, it started to download the files, reached 100 per cent, then it started 'patching files' AND it stalled. I left it 'patching files' for 20 minutes with no movement on the progress bar, then i forced shut down (no other options) and i tried to restart, it got stuck on the grey screen with the rotating 'loading' icon, it rebooted without prompt, and it got stuck again on the grey screen with the loading icon. I've tried safe boot and PRAM. Neither has made a difference. I even left it on the grey screen with the icon rotating for 4 or 5 hours and there was no change. Indeed, the sleep light (bottom right edge of macbook) came on after 20 minutes of the rotating icon.

Can anyone let me know if this has happened to others? I noticed other discussion about the external screen display problems but nothing like my problem. I'm really concerned I will lose all my work...