What causes a computer to shut down and immediately restart itself when you are in the middle of something?

There is a problem with the operating system, or with data stored on the hard drive. If the machine is running Windows 98, right click on the icon for the hard drive, and open the 'Tools' menu. Select 'Error checking' and proceed. If the computer is running Windows XP, go to the control panel, select 'Maintenance' and look in the upper left corner for 'System Restore'. Point at that and click, and then select 'Restore system to an earlier time.' You will not lose any data, unless the hard drive is severely messed up. You should already have backed up everything important on the hard drive to a CD-ROM, or some other storage media. If this does not help, you need to have a technician check out the machine. Such a person can determine if the Central Processor Unit or the power supply have gotten so dirty that they are overheating, or if the software is corrupted.