Question: I like having my laptop run as long as humanly possible when running off battery, even if it means huge a compromise in performance. 

Currently, whenever I'm on battery I have to manually switch my power plan to "Power Saver", and then switch back to "High Performance" when I'm plugged in. 

Is there any way I could have my computer do this automatically?

Answer: 
There's nothing built into Windows that will allow you to do this, but I've seen a couple good 3rd party applications.

[Aerofoil](http://silentdevelopment.blogspot.com/2008/09/aerofoil-144-rated-47100-hot-100-vista.html) is my preferred of the two. It's very simple; all it does is switch your power plan to "power saver" and disables the Aero glassy desktop manager when you switch to battery, and does the inverse when you plug back in. It works great in Vista and 7.

[Vista Battery Saver](http://www.codeplex.com/vistabattery) has a lot more features and can give you a bit more fine grained control. However, I've found it to be a bit less reliable -- doesn't seem to work 100% of the time for me. Also it doesn't work in Windows 7 at all -- it gets stuck halfway through installing.
Doesn't each power plan in Vista/7 allow you to configure settings for both AC and DC power? A/C = plugged in, D/C = battery. Just adjust as you see fit, and you won't even need to switch power plans.