Cause - Various factors
If the PC is restarting (instead of turning off) when choosing Shut Down from the start menu, a variety of things could be causing this. Mostly, the issues center around legacy hardware and software compatibility issues. Keep in mind this could be a global symptom, as Windows XP will automatically restart when there is a system failure.
1. The first step is to disable reboot on system failure. Right-click My Computer, click Properties, then choose the Advanced tab.
2. Under Startup & Recovery, click Settings. Under System Failure, uncheck the box in front of Automatically Restart.
3. Automatic Restart on System Failure can also be disabled from the Windows Advanced Options Menu; access this menu at boot-up by pressing F5 repeatedly. If a stop error is indeed causing the issue, make note of the error code and use a search engine to find solutions.
4. If it is a specific hardware or software issue, troubleshoot with the provider of the troublesome piece.
5. If no stop error occurs, the next step is to check the "wake on" power settings in the BIOS. Refer to the specific model's instruction manual for more information; manuals can be downloaded from the downloads section.
6. If any USB, LAN, or other "wake on" settings are turned on, this can cause the system to reboot (instead of turning off) when choosing Shut Down from the start menu.
7. If all "wake on" power settings are off, disconnect all devices connected to the machine.
8. Using a process of trial and error to discover the problem component, reconnect each device one at a time.
9. Once the offending device is identified contact its respective Tech Support Dept. for possible solutions.