You'll have to remove the Blasgter worm.
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The following is courtesy of Carey Frisch:
Apparently, your computer has an unwelcome visitor
(VIRUS) known
as the WM32 Blaster/Lovsan WORM. It only takes a split
second for
this WORM to infect your computer if a firewall is not
enabled and you
have an active internet connection.
It entered the computer through the internet connection
because the "firewall"
was not properly enabled and essential critical updates
were not installed from
the Windows Update website. For instructions on fixing
the problem, visit:
What You Should Know About the Blaster Worm
http://www.microsoft.com/security/incident/blast.asp
Remove W32.MSBlast.Worm
http://www.kellys-korner-xp.com/xp_qr.htm#rpc
Blaster Worm: Critical Security Patch for Windows XP (32-
bit version)
http://microsoft.com/downloads/details.aspx?
FamilyId=2354406C-C5B6-44AC-9532-
3DE40F69C074&displaylang=en
If your computer is constantly attempting to shutdown
or reboot, quickly go to:
Start > Run and type: CMD , and hit enter.
This opens the Command Prompt window.
Then type: shutdown -a , and hit enter.
This should halt the rebooting problem.
Then immediately turn-on Windows XP's built-in Firewall:
How do I turn on the Firewall?
http://www.andyrathbone.com/tips/firewall.html
**** IMPORTANT ****
Visit this web site if you experience difficulty
installing
Windows XP Security Patch 823980:
How To Fix The Cryptographic Service Error
http://www.updatexp.com/cryptographic-service.html