usually when a pc shuts down with no warning there are a few possibilties of whom most are "self protection" here are a few common causes ..

over heating :check all your fans work and that every fan runs without squeeling noises wich could indicate a fan failure , make sure your cpu fan is not limited in the bios to a certain speed but adapts its speed to the heat (modern hardware usualy support this )

make sure your cpu fan and heatsink is fastened propperly ..

make sure you have clear airflow and not blocked off by dust etc..

hardware : if you have added any new hardware , remove it and try again .

remove all non essential hardware and try again .

a hardware failure could produce this but usually this would happen not to just one piece of software .

operating system : start your pc enter the bios mode and check the heat statistics of your pc or make sure no errors , leave the pc running a while in bios if it doesnt shutdown then it is likely that the os (windows) are experiencing problems the cure is to remove all software do a clean install (usually) ..

power : sometimes your pc might have just enough power to keep running untill a little more is expected from it like a cdrom or some fans or something speeding up and which when it oversteps the power supply units power rating it will power down with no warning , a failing power unit also produces this ..

check to see you do not have a auto shutdown setting ..

if the problem only happens with certain software you might have a virus infection ,yes theres a virusses and malware that does that , update your virus software and scann your pc .

ps. i would start on the virus check ..
also scan your disk for errors.

almost forgot , overclocking is also a cause of over heating .
running memmory at the wrong speed also causes random failure due to corruption but ussually does not just power down a pc