Windows 7 allows you to change the power plan that your system uses, which determines the balance between power usage and performance. For example, on some mobile devices like netbooks it may be desirable to select a power plan that maximises battery life.
To change your power plan, type in powercfg.cpl to the start menu search or the run dialog box (Press Windows Key + R to access this) and press enter. Once the power options page has loaded, you will be able to select one of the pre-configured power plans (often High Performance, Balanced and Power Saver):
Simply click on the plan you wish to use and close the window. If you want to customise the settings further you can click "change plan settings" then "change advanced power settings" to modify an existing plan, or create your own power plan by clicking "create a power plan".
It is important to view the advanced plan settings on mobile computers (like laptops and netbooks), as this is where you can specify a difference when the device is plugged in to the mains or running on battery power. For example, you may wish to run a power-saving plan when running off internal power, but high performance once it is externally powered.