Because this is anything but cutting edge technology its probably well documented all been done many years before.
Back then, there weren't usually a lot of over-clocking option. Very few motherboard did offer manually adjustable bus speeds. Instead, most of the time you had a fixed series of frequency for the front-side-bus : 66/100 and 133Mhz, sometimes 75,83,103 and 115 too. In fact a lot of Pentium-III era motherboard don't even have a BIOS settings for bus speed but rely on hardware jumper to select a given frequency from the bus.
Probably, on a portable computer, the BIOS doesn't offer any option at all. With some luck, the clock generator (PLL) can be programmed using the SMB/I2C bus. Sadly, SpeedFAN seems to work only with a couple of more modern boards. But there are old software, like
CPUFSB that have a lot of settings for legacy motherboards. (Supports my old ASUS P2Bs)
Either find the motherboard model in the list, or try to identify the PLL chip and search it. But don't expect a lot of configuration option. Most probably you'll be able to select the standard official speeds (66/100/133) and maybe a couple of other additional frequencies (75, 83, 103, 115 and 124 are popular steps).
Pay attention that, back then, there wasn't a separate frequency for the front side bus and the memory. Both ran at the same speed, so you must check that the memory SO-DIMMs support the selected speed.
Also, Coppermines are done using a huge 180nm process. So they tend to heat a lot when overclocked, which may become an issue in a not-so-well-cooled device as a laptop. So you have to find a way to better cool the CPU if you want to overclock more than small steps (More than 100->103). On the other hand, once properly cooled, you can easily expect at least 10% overclocking, maybe even more.
Could anyone direct me to a utility that runs in C ?
I don't understand what you mean by "
Runs in C". Do you want C-source code examples to code you own Coppermine-overclocking solution ?
"lm_sensors" have a nice collection of code for chips (mostly sensors) on the SMB/I2C bus. It used to have a module called "icspll" in older versions that might be a good starting point for writing a PLL driver.
But I think, what you need best is to identify the PLL chip you have and download its spec from the manufacturer's website (probably an ICS chip).
PLLs aren't very much complex (specially the old one that only provided a fixed number of frequency steps) and you could probably set the bus speed you want with only a couple of writes (using code from other lm_sensors' SMB/I2C chip modules as a starting point).
