Shutdown after restart

I'm just upgrading to Leopard and have encountered a serious problem. After installing Leopard (upgrade) the computer restarts. As soon as the grey screen appears and the spinning gear starts the computer shuts down (turns off). This is the first time I have encountered a problem using the Upgrade option since starting with Tiger. Having a problem didn't surprise me as I'm sure I have many orphaned files hanging around. After trying several solutions I restored my original OS disk. I have complete daily backups to get me back into service.

To test if I had a hardware problem I decided to use a spare firewire hard drive as a test system. I could play with it as much as I wanted without effecting the use of my computer. I erased it and then installed Leopard. I walked through the basic setup program but skipper the migration assistant. When I booted from the firewire drive and everything worked OKOK. This eliminated a hardware problem. This meant that I could do an archive and clean install to resolve my problem. However, I was interested in what the Migration Assistant would do. I had a clean drive with only Leopard on it that would eventually be erased. I went back and ran Migration Assistant selection the necessary options and let it run. After a little more than a hour it finished. I restarted into Leopard and everything was as usual and I was back in service. After playing with it for a few hours it appears that I have no problems.

Question -- Does anyone have an insight as to what the original problem was? What loads in those first few seconds of startup?

How about some comments on the Migration Assistant solution.

Thanks