Errr... if XP can down clock, I'm sure Vista can.
Even the power schemes in power options can or will have an effect.
I'd disable C1E and leave EIST on. That way by using the power options, if you set it for min power management, it will actually force everything to run lower at idle. If you set it to always on, it should bump it up to max, even at idle.
Thanks to Zorg, I figured that out.