System Freezes after couple of hours !

I have an HP Pavilion 8290 with Windows 98. For the past couple of weeks my system freezes(hangs!) with a blank screen and a mouse cursor if left unused for sometime. Currently the energy saver settings are 1) Turn off monitor after 30min 2) Turn off hard disks after 2hrs.
After the system hangs I have to turn off the power switch OFF and ON to reboot the system. Any suggestions ? Thanks