Unlikely going to last 7 hours.
Generally, you change clock settings in the BIOS, and whether you can underclock or not depends partly on the BIOS.  Just go in and lower the core clock speed as much as possible - your core clock is running at 166MHz, so lower it as much as possible.

Do you have EIST enabled?  If so, it should fluctuate your multiplier automatically as well.  Make sure you have the "Minimal Power" scheme set in Windows power management.