If you wish to underclock the CPU dynamicaly to save power there is a setting in the BIOS called SpeedStep.  When enabled and not under load the CPU will underclock itself to use less power and when it needs more speed it will clock itself back to full speed.  When disabled the CPU will run at full speed.

This option is loacated under the CPU Features of your mohterboard's BIOS.  The CPU features can vary in location depending on the board but it is typicaly under Frequency/Voltage control or FSB & Voltage control.