Please Help!!!!!!

Ok so I think I may of messed up bad. Heres the story.

I was sitting home, doing work on my macbook, when I noticed that the charger light was lighting up, and that my macbook said it wasn't plugged in even though it was. This seemed odd and just decided to restart and hopefully it'll fix problem.
As I was going to restart, the software update popped up and said I need to restart my computer to update to 10.5.1, the new itunes, etc. I figured "o perfect" so I chose ok and let it restart. The software update never did quit and thus my macbook didn't restart. I shut if off manually and when I rebooted and tried to log back in, the mac osx setup assistant keeps appearing. I tried to complete it, but when I do, it takes me back to the log in window, which I try to log in again, but brings back to the setup assistant.
What do I do?? Help!!!! Thanks.