System always hang…

Hi,
I was wondering if anybody can give me a pointer or two regarding an intel pentium mmx 166mhz system. This system always hang after about 5-10 mins running an application from the hard disk, but not when it is in idling condition or when installing applications from the cd-rom drive.
Power supply is having an output of 230W and its fan is still turning whenever the system hang. I also notice that the CPU fan is also turning whenever the system hang. Pressing ctrl, alt and delete does not help at all, I have to power off and restart system again.
Any suggestion is greatly appreciated.
Thanks.