Your notebook's display and hard drive are its two biggest battery hogs. But you can control them by using a Windows XP Control Panel utility called Power Options. (Earlier versions of Windows offer similar power-saving options in the Control Panel, too.)
Go to the Start menu and click Control Panel. If you're in Category View, click 'Performance and Maintenance,' then open 'Power Options'. If you're in Classic View, just double-click 'Power Options'. On the 'Power Schemes' tab, select 'Portable/Laptop' from the top drop-down menu. In the 'Settings for' area, you can tell your notebook to act differently depending on the power source. Save your settings by clicking the 'Save As' button and clicking OK.