On my laptop that runs Vista (with the Aero Glass enabled) I receive about 4-hours of battery life on full screen brightness. I would say that is pretty darn good, but I can often squeeze out an additional hour or two if I use the free Vista Battery Saver.
This program is designed very well, and with it running you’ll be able to disable the Aero interface and the sidebar if your battery level gets too low. The Aero interface is claimed to reduce your battery life by 1-4% of what it would normally be, so disabling it along with the sidebar should definitely give you some extra time when you need it.
Also, a new feature in Vista Battery Saver lets you choose a different power plan for Vista anytime your computer isn’t plugged into an outlet. This is nice because you can send Vista into Power Saver mode when using your battery, and have it automatically switch back to a more performance-focused state when you plug back in. And from personal experience I can get an extra hour of battery life just by using the Power Saver mode, so this is a very welcomed addition to the application.
All you have to do is download the Vista Battery Saver, install the program, and set it up once. From then on it will automatically start with Windows, and will quietly do all of the work in the background while running in the System Tray.