tomas, the first thing you need to do is go to HP or the manufacturer of the motherboard and get whatever information you can on jumper settings for changing the clock multiplier, FSB (front side bus) speed, and CPU core voltage. If you look closely at the motherboard you might even notice markings or tables that give the different possible settings for different various jumper positions. Note- if you decide to run a FSB faster than the stock 66mhz you'll also need to make sure that whatever type of RAM you are using can handle it (preferably SDRAM as it is more tolerant to higher frequencies than EDO ram).
Once you have done this you can then begin to slowly increase the operating frequency of the CPU by repositioning the proper jumpers on the motherboard. Keep a close watch on CPU heat production and try to keep the CPU temperature from exceeding say, 130 degrees farenheight to avoid burning up the CPU. The Intel 200mmx is usually quite tolerant to overclocking. Incedentally, the stock FSB and multiplier settings for this CPU are 66mhz FSB and 3.0 multiplier respectively.
Try increasing the FSB to 75mhz and the clock multiplier to 3.5 . If you discover that the highest possible clock multiplier for your motherboard is 3.0, then your only option will be to adjust the FSB speed if possible. In any event, have fun.