Hi Nadia, I'm afraid you are infected with the "Blaster Worm". The symptons you describe are a classic example of such an infection. To stop your computer from shutting down, if you click Start, Run and Type:
shutdown -a and Click OK, this should terminate the shutdown. What you then have to do is install a a "patch" from Microsft, you can get the patch from here >>
http://tinyurl.com/is2b
<< Now what you have to do is, well delete this infection, Norton should have picked it up so i would ask have you updated the Virus definitions? Anyway there is a removal tool from here >>
http://tinyurl.com/jqdf
<< or here >>
http://vil.nai.com/vil/stinger/
<< If you have system restore ON then you will have to turn it off as the removal tools will not be able to delete an infectiom the System Volume Information folder (these are protected operating system folders).