There are three causes why Xp reboots when shutdown.

1.Recovery settings

2.Software incompatibilities, including driver issues

3.BIOS problems

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.Clearing the Automatic Restart is only the first step to solve the
problem.

How to Fix Recovery Settings1. Right Click your my Computer Icon on your Desktop. Then click properties.
You can also use this method.
Click Start, click control panel, and then click System.
This box will appear.

2. Then click advance button.

3. In start-up recovery, click settings,

4. Uncheck automatically restart.

Then Click Ok.Click Ok again. As simple as that and your Computer is back normal operation again

If after disabling automatic rebooting, the problem still occurs, look for the software and driver issue. Maybe you have installed a new program or a driver that is incompatible with Xp.If the problem still occurs, the next thing to do is to check your Bios. Maybe your Computer manufacturer has updated their Bios. Virus and Hardware failure may
also be the cause.