i log on onto my windows xp and it automatically logs me off!

I was updating my norton antivirus and the laptop turned itself off,
so i restarted it and when i did i came to the user log in page and i
put in my password and i was logged in and then after a few seconds it
automatically logged me out!! i then tried to access the guest account
but it did the same thing. i have tried to restart the laptop, and i
left it off all night, but even if i try to follow other methods like
to re install xp, because i cant get onto my account i can't do
anything, and i can't even get to the start menu!! someone please
help, it's got all my coursework and work on the laptop and i need a
solution!!! please!!