Window NTAuthority/Shutdown

Sounds like a nice virus... either blaster, gaobot, something to that fact...

You need to disable system restore... boot into safe mode with network support. Run and update your anti-virus.

Use lavasoft's Adaware and / or Spybot S&D to clean the rest of your system.

Download latest service packs from microsoft...

Reboot normally, re-activate system restore. You should find a nice worm or files that have been screwing around in your computer.

A quick fix if it comes up is: goto start>run>type in
shutdown -a
then click ok, this will kill the shutdown process, untill you can get the patch. Once you do have the patch, make sure you're not online in the midst of installin it. If you have dsl or the like, unplug the line from your rig.

You have the blaster virus. Follow the directions above and remove it from your system, preferably while not connected to the internet. Then when clean and functional redo system restore, and...create a restore point manually. Then download the megaload of XP patches. XP's update wizard will find them all for you. Be aware that some must be installed singly and in certain order. This will also require rebooting many, many times. Once you're system is cleaned and patched, make sure that Internet Connection Firewall is enabled (if you don't already have a program such as ZoneAlarm) Here are 2 sites that go into this more.

http://www.faqs.org/rfcs/rfc1831.html
http://mvps.org/winhelp2002/unwanted.htm