You are most likely infected with the Blaster worm virus.
To stop the rebooting/shutdowns, right click on the task bar, choose task
manager, processes tab, look for msblast.exe. Highlight it and click end
process. Then turn on the xp firewall.
To turn on the firewall: control panel, network and internet connections,
network connections, right click your connection, properties, advanced tab,
check the protect my computer box. Do this as quickly as you can once the
desktop comes up. Then visit the sites below for the removal and patch info.
Symantec: removal info and removal tool
http://securityresponse.symantec.com/avcenter/venc/data/w32.blaster.worm.html
Network associates: removal instructions
http://vil.nai.com/vil/content/v_100547.htm
Also make sure to follow the links to the Microsoft pages for the patch or
visit windows update for the patch after you remove it.
http://support.microsoft.com/?kbid=823980
http://www.microsoft.com/security/incident/blast.asp