Do you have a UPS ? Have you configured its settings in the power options ?
The reason I ask is that I just configured some settings to my UPS in the power options and i had the same problem like you ....without warning the comp would shut down ...not even reboot. It took me a whole day to isolate the problem . A full re-install followed and then I didn't configure the UPS and now its fine .
Just one of the weird things that can happen .
Other things could be -
1. OS problem
2. H/w problem
3. Power problems
Also don't forget to run a virus scan with NAV - with updated VDFs . If everything fails ...then just open the cabinet shout at your processor and abuse it verbally ...close the cabinet - plonk the tower with a hammer and see if it works ...
just kidding ...