Win 98 Freezes

I have a Dell Optiplex 500Mhz P3 computer with 64 mg of RAM. Intermittantly it will lock up. The only way to “fix” it is to power it down by holding in the power button for 10-15 seconds. It can do this when only one program is running and the computer has only been on for 5 minutes, or it can do this after the computer has been on for 3 days and has 4 open programs. I stopped the graphic accelorator, but it had no affect. Does anyone have any ideas, hints, etc? Any help would be greatly appreciated.