Try booting the system to Safe mode.
To boot to safe mode:
Restart the computer. As soon as the BIOS has finished loading, F8 key on your keyboard. When Windows Options menu appears, scroll to select the Safe mode menu item, and then press Enter.
Other method to boot to safe mode can be found here
Once you managed to boot to Safe mode, change the computer startup and recovery settings by right-clicking "My Computer" then select Properties. Select Advanced tab. Under Startup and Recovery, click Settings. Uncheck the box for "Automatically restart" under System Failure.
Reboot again normally, see if it will reboot again by itself. If you managed to make the system boot normally, proceed to Events Viewer to find out what is the caused.
If the system failed to boot normally after doing the above, you might want to boot the system to "Last known good configuration" See - http://support.microsoft.com/default.aspx?scid=kb;EN-US;307852
Running XP--my son's computer keeps shutting down when trying to boot up, as soon as it gets to the welcome page.