Shut down problems with computer?

my desktop computer - Window XP operating system, approx. 7 years old is turning off on me with no particular reason, meaning whether I am reading e-mails, listening to music, watching videos etc. It does however crash more often & quicker if the system is under load such as videos, music, multiple pages open etc. it shuts completely off & I have to push the start up on to reboot. it has been doing this now about 3 months, I'm over it. I am wondering if possibly the cooling fan is bad, I have a fan placed behind it that I keep running on it to help cool it. It seems to help some but not much. Sometimes it will run a few hours if your lucky but usually I'll get about 30 min. to an hour then it just constantly keeps shutting down after that time period like every few min. HELP PLEASE COMPUTER TECHS, GEEKS, GURUS. ?? virus, another type bug, heat (over heating) what ???