Processors clock down to conserve energy and produce less heat. Even enthusiasts leave their processors to clock down rather than running at full tilt. To run full tilt is unnecessary, and has been since the feature was introduced long ago now. If the processor load isn't above a certain amount, there's little to no benefit to having the processor run at full speed all the time; these tasks are likely so light (such as playing a DVD, casually browsing the Internet, sitting at the desktop, etc) that the increased processor speed would go unnoticed.
I know Windows 7 offers an option in the advanced power settings (where you customize power schemes) that allows you to define the minimum and maximum processor speed (in percentage); whether the setting will be observed in the manner you desire is unknown. But...once again, there's really no reason to force the clock speed to be at the max at all times.
The motherboard may also have a setting that controls this. This depends on the manufacturer and the decision to implement features in a certain manner.