Hi,
There is every chance that someone who knows what s/he is talking about will come along to help.
In the meantime, I'll be glad to give it a shot!
It seems implied, but your message doesn't state this specifically, so I'm going to ask. Can you log on the first time successfully without an ensuing reboot as ANY user, or only as yourself? If you can sign in as ANY user the first time, and then a logout / logon cycle results in a reboot, that's probably going to have a very different cause from the situation where you can only log on as yourself or Administrator or whatever, and a logout / logon cycle to another user results in a reboot. Also, what happens if you logout as a user then logout / logon as the same user?
Regards,
Jim
Oops - Edit: I forgot to ask if you get any error messages in Event Viewer's System Log or Applications Log.