Windows Sidebar uses idle processing power, when the CPU should/could be in a low power state - this is not ideal for a notebook when running on battery.
Windows Vista will show warnings in the Power Options like "Your current plan may reduce battery life" or "Your current plan may reduce performance" if you use (respectively) high performance on battery or power saver when plugged in. Aerofoil can switch these two plans for you automatically so these messages will never appear.
Installation and usage: Download Aerofoil from here, unzip and install it. It will put a small configuration icon in your system tray. Just right click on it to select the options such as Enable/disable Aero Glass, Auto Sidebar and Auto Power Plan when your Notebook running on battery. This tool also provides a quick hibernate shutdown option.