First eliminate whether it's virus related?
http://www.microsoft.com/security/incident/blast_faq.asp
Blaster Worm FAQ

Wow a 6 year computer veteran who hasnt heard of the
blaster worm or who doesnt know how to properly secure
their computer wow......

Your computer is now infected with the W32.Blaster.Worm or
one of its variants. This happened because you have not
been using an internet connection firewall and have
apparently neglected to install the critical updates
available at the Windows Update website.
-----------------------------------------------------------
-------
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:
http://www.microsoft.com/security/protect/
(To enable the built-in firewall, go to:
Control Panel, double-click Networking and Internet
Connections, then click Network Connections. Right-click
your connection, then
Click Properties, and on the Advanced tab, click the option
"Protect my computer and network..." Note: the built in
firewall only monitors incoming traffic not outgoing (ie
spyware, trojans, etc.. you may have on your system).)

Special note if you use AOL:
America Online installs its own connection settings that
override
the ones that come with Windows XP.  America Online's
connection settings don't include a way to turn on Windows
XP's
built-in firewall.

What You Should Know About the Blaster Worm and Its
Variants
http://www.microsoft.com/security/incident/blast.asp

A tool is available to remove Blaster worm and Nachi worm
infections from computers
that are running Windows 2000 or Windows XP
http://support.microsoft.com/?kbid=833330

A security issue has been identified that could allow an
attacker to
remotely compromise a computer running Microsoft Windows
and
gain complete control over it. You can help protect your
computer
by installing this update from Microsoft.
http://www.microsoft.com/downloads/details.aspx?
FamilyId=2354406C-C5B6-44AC-9532-
3DE40F69C074&displaylang=en

Above courtesy of MVP Carey   
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

***Install a good firewall. ZoneAlarm is a free one you
can install.
Install a good anti-virus program making sure you keep
it's definitions up to date! ***
- - - - -  - - - - - - - -
Microsoft Security Bulletin MS03-39
http://support.microsoft.com/?kbid=824146

What You Should Know About the Blaster Worm
http://www.microsoft.com/security/incident/blast.asp

Protect Your PC
http://www.microsoft.com/security/protect/default.asp

W32.Blaster.Worm  a.k.a. W32/Lovesan.Worm
http://www.symantec.com/avcenter/venc/data/w32.blaster.worm
.html

W32.Blaster.Worm Removal Tool
http://www.symantec.com/avcenter/venc/data/w32.blaster.worm
.removal.tool.html

W32.Welchia.Worm a.k.a. W32/Nachi.Worm
http://securityresponse.symantec.com/avcenter/venc/data/w32
.welchia.worm.html

W32.Welchia.Worm Removal Tool
http://www.symantec.com/avcenter/venc/data/w32.welchia.worm
.removal.tool.html