underclocking or CPU frequency scaling

[thufir]

I have an older desktop:

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

and would like to set a "max" for CPU usage to prevent overheating.  
Generally, it's fine, but for some processes I want to set a sort of 
"nice" so that the CPU doesn't get pegged.  Longer compiles would be fine.

Frequency scaling and underclocking can only be done through supported 
hardware, generally the BIOS?



thanks,

Thufir