Spontaneous PC shutdowns

System shutdown could be caused by software that hsa been downloaded recently
Boot into Safe Mode and run diagnostics,scandisk,defrag
If you can you may want to restore system back to an earlier date before this problem occurred

Check the system also by running your anti virus and spyware tools

When the monitor goes off try pressing the power button once (not holding it down) and see if the HDD light flashes and the PC switches off (As if it was doing a manual shutdown). If this works try a different video card or even try reinstalling the current drivers for the video card you have.

If the card is faulty, it could be that the video card is sending the monitor into a power saving state prematurely.

Will it run if you don't start Windows?

For a start try using a Bootable Floppy or CD and see if the system continues to run correctly.
You can try a Live Linux or better still The Ultimate Boot CD from

http://tinyurl.com/3jnpy

To run some diagnostics. At least that way you can see if the problem is being caused by the hardware or Windows and then start to begin to repair the problem.

Col

1} Reset your BIOS.
2} Unplug PC from power supply and hold power button for 10 seconds.
3) Swap memory sticks.
This worked on HP DC7600