Computer Shut Down Problem

I’m currently running with Windows XP Home Edition. Before hand I had Windows ME. Now my problem didn’t start until I upgrade to XP. It seems that after I open about 5 programs my computer decides that it’s tired of working and completely shuts down. This includes the power supply. I’m not sure if it’s a short, if my computer is running hot, or if I’m not running with enough memory. Is there a solution to my problem?