If your PC/laptop is randomly shutting its self down, you have the sasser worm.
This worm will not harm your computer, but it will be very frustrating.  The way to keep
this worm from infecting your machine is to keep your Microsoft Updates up-to-date.
If you see a globe icon in your task bar (where your clock is) always click it and install
the updates.  Those who have installed updates will not be affected by this worm.

To remove it, do the following:

1.  Start your PC in safe mode To start in safe mode, Shut down PC, turn it back on,
     and when it gets to the Windows XP splash screen (where there is a black screen
     and it says Windows XP Professional) Press the F8 key.

2.  Press the F8 key.

3.  Chose Safe Mode with Networking.

4.  Let machine boot up. 

5.  Once at desktop, come back to this site and download the Removal Tool and
     save it to your Desk Top.

6.  Click the tool, and let it scan.

7.  Once finished, the worm will be off your machine.