Is it shown on a blue screen. If so you may have a hardware failure.

There are many reasons that your computer could be shutting off. Without knowing a bit more, it is hard to pinpoint the problem. However, you could try some of the following. 1) Start your computer in Safe Mode (assuming you have Windows) by holding down F8 while the computer is in the process of starting up. This will start the computer with the minimum of services running, and will often (but not always) disable any virus or spyware that is installed. You can then run a virus scan of your complete hard drive, and see if there are any hits. If you need an antivirus program, AVG Free edition is good, and as the name suggests, free. 2) It could be a problem with your power supply. This is the part inside your computer that takes the power from the outlet and turns it into the low DC voltages that your computer needs to function. After years of use, or after a power surge or some such thing, it can start to fail at random. This is less likely to be the problem since you are seeing error messages. 3) It could be a problem with your OS installation. Windows in particular has problems with corrupted registry settings, or system files. If your crashes are indeed random, this again is not likely, but the could be unpredictable to the average user and still exhibiting a pattern that is relevant to Windows. The solutions are as follows - install a good (updated) antivirus program and run a full scan, in safe mode if possible. If this fails, try repairing/reinstalling your OS. Finally, try a new power supply.