So I haven't had this problem until recently, but my homebuilt PC freezes up occasionally. When I say this, I mean it just totally locks up. No crtl-alt-del, no alt-tab, no nothing. Everything just goes dead. Fans keep spinning, but the signal to the monitors is also lost.
I just installed Windows updates, my video card is on latest drivers and from what my AV is telling me, I don't have any viruses. I have more than enough power and the cooling should be sufficient.
how old is the rig, its possible your gpu is experiencing early failure signs (although each time ive had a video card die it would display random fractals first)
defragment your HDD if you havent recently it could help,
also double check your power lines while the computers in use check they are not too hot and you dont have too much connected to one surge protector, it could be something as simple as having too much juice flowing on one electrical fuse inside your home, (i have a separate circuit for tv/computer used to cause the house to brown out) and the final thing could be something as simple as corrupted file, my current pc tends to randomly crash in full screen but in winodw mode only the game crashes not the computer.. best of luck ill post more if i come up with something else later on