underclocking or CPU frequency scaling

[compdoc]

> motherboard:  SiS-661GX
>CPU:          Intel(R) Pentium(R) 4 CPU 2.53GHz

That cpu consumes only about 62 watts, so it shouldn't be hard to keep cool.
Make sure the cpu fan is running at its rated speed, and make sure the heat
sink is not clogged with dust. 

SiS was a low cost chipset, and possibly the slowest chipset on the market.
Being as old as it is, it's doubtful it has any sort of power saving
features in the bios that keep the cpu running cooler.