I remember being online when the computer network at my university got infected with the blaster virus, and I remember they were all switching off & rebooting. Your's seems to be just switching off, though, so I'd not rule out a possible hardware problem as well (in the computer and/or power supply or cables ... check to see if all looks ok when you switch off). It could be overheating, though it seems strange for a computer to get overheated in winter - unless you have a heater going strong enough in the room. Definitely do the virus scan first, & see what that comes up with.