Upon shuting down PC, will shutdown & reboot automatically

Sounds like you're throwing a "fatal error"--the legendary BSOD.
To find out for sure, follow these steps to disable automatic rebooting for fatal errors. This will at least allow you to see the issue.
1.
Right-click My Computer, and then click Properties.

2.
Click the Advanced tab.

3.
Under Startup and Recovery, click Settings to open the Startup and Recovery dialog box.

4.
Clear the Automatically restart check box, and click OK the necessary number of times.

5.
Restart your computer for the settings to take effect.

If, after doing this, you get a Blue Screen of Death, just write down the Stop Code and the hex value that it shows (something like 0x1a).

If that's not the culprit, have you installed anything new recently? Roxio is a known culprit for auto-reboots.

Failing that, it could be a hardware issue, or a BIOS incompatibility. You can try updating firmware to troubleshoot that.
If none of these works, a repair reinstall of your OS (you won't lose any data) will probably take care of it for you.

http://michaelstevenstech.com/XPrepairinstall.htm

That link's pretty good on how to do that. Good luck, chief! Let me know if it's a BSOD.

While there are many potential software related causes for a reboot as outlined in the first answer, it could also be a hardware issue.

When it reboots, just after the monitor goes blank and POST starts for the reboot, press and hold the power button. If the system shuts down, then you do have a software issue and you can start running through the suggestions in the first answer.

If it does not shut down, you may have a hardware issue. I have had a number of motherboards where the power control circuit has failed and no longer tells the power supply to shut down.

Other hardware issues include having Wake-On LAN enabled and a network device pings the computer while it is shutting down, prompting a reboot.

Chas

Shut down or automatic restart? Any error messages? As mph asked: do you get a BSOD (Blue Screen Of Death)? If so, what error message and code is displayed?
Have you already tried a System Restore to a point in time before this problem started?

www.microsoft.com/windowsxp/using/helpandsupport/getstarted/ballew_03may.mspx

Recovery Settings

One of the things that is quite different about Windows XP compared to Windows 9x (9x is shorthand for Windows 95, Windows 98, and Windows Me in all their various versions), is that one can control how it responds to certain critical errors?those that cause the infamous Blue Screen of Death (BSOD). In Windows XP, the default setting is for the computer to reboot automatically when a fatal error occurs. If that fatal error only occurs when you're shutting down, the system reboots automatically.

If you haven't changed any of the system failure settings, you should be able to see the error by looking in the Event Log. But a better long-term solution is to turn off the automatic reboot so you can actually see the error when it happens?chances are it will tell you enough about itself to let you troubleshoot further. To change the recovery settings to disable automatic rebooting:

1.Right-click My Computer, and then click Properties.

2.Click the Advanced tab.

3.Under Startup and Recovery, click Settings to open the Startup and Recovery dialog box.

4.Clear the Automatically restart check box, and click OK the necessary number of times.

5.Restart your computer for the settings to take effect.

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