Computer started freezing up

Hello,

I have a older Windows XP desktop and recently it started freezing up, but mouse still moves, but cannot click on any of the icons. I am unable to turn computer off the normal way and have to wind up unplugging it and it comes back to normal immediately. I can use the computer as usual for a while and then it freezes again. If I unplug and plug back in and leave it alone, the screen goes black except the lower left corner start button and the lower right corner time.