Does the machine hang after a certain period of time or does it vary? If it's variable, it sounds very much as though it is overheating. Most PC's these days are designed to power down if the CPU and/or motherboard chipset overheats and won't let you boot back up until the temperature falls, though machines without temperature monitoring facilities tend to just hang.

It may be worth running a full system scan to check for viruses, particularly if it hangs after a certain operation. However, the symptoms would seem to suggest hardware problems rather than software.