you can stop it shutting down by running a command in a MSDOS window.
shutdown -a
This is only a temporary solution. You have a virus still. When did you last update the virus database in Norton? I'm not a fan of Norton but up to date it should still remove the virus. If you haven't got an up to date subscription then uninstall Norton, download AVG from
http://free.grisoft.com/, install it and let it do all its updates. then let it have a go.
You also need to make sure you have a firewall on that is correctly set up. If you haven't, get the basic version of Zone Alarm from Zone labs. Whatever you do they will try and sell you the paid for version. Don't bother, the free one does the job just fine. When its installed please make sure that you don't just click willy nilly allowing all and sundry access. Make sure you know what programs you give permission to access teh net, any you aren't sure of click deny.