There are many reasons why your Windows XP keeps restarting.
There are numerous things you can do to isolate and identify the problem.
But one of the most common causes of Windows operating system instability is the presence of errors in your Windows registry.
   The Windows registry is a database that resides on your computer's hard drive.
It is the master database that the Windows operating system uses to store all configuration information about every piece of hardware installed in your machine, every peripheral or USB device ever attached to your machine, every software program ever installed on your machine, every file type ever downloaded or executed on your machine, every user account associated with your machine, and all of the core operating system behavioral characteristics.
   Whenever you add and remove hardware, add and remove serial or USB devices, install, uninstall, download, create and manage user profiles, or even simply run programs on your PC, the Windows registry is accessed.
Information is either read from it, or information is written to it.
   Over time, this database can grow to become very large and bloated.
This causes searches against the registry to slow down.
Over time, errors can creep into the registry as well.
If you move files around, rename them, uninstall or reinstall software or hardware, or reboot your PC without shutting it down properly, referential integrity errors can result, due to invalid link that should have been deleted but weren't, entries that reference hardware or software components of your system that are no longer there, entries that weren't properly updated to reflect changes in your computer's state, these can all cause your registry to become out of synch with the true state of your computer.
   If your PC happens to be searching for information or needing to write to the registry at any given moment, and the registry happens to contain errors or happens to be extremely unoptimized, as described above, then your PC will inevitably become unstable, and can lead to spontaneous crashes and reboots.
The most straightforward remedy for this type of situation is run what is known as a registry repair utility.
What this does is scan your registry for errors and it actually corrects them.
  The result is a more robust, more optimized, more stable, and more efficient Windows operating system.
Running a Windows registry scan is equally as important as running anti-virus, anti-spyware, or even firewall software on your PC.
   The greatest threat to your computer's security may not necessarily be a hacker, but may be your own PC's performance degradation.
This could be the cause of its own undoing.