If you've got a laptop running Vista and it seems that your laptop battery never seems to hold a charge between the times that you use it, don't blame the battery -- blame Vista!
The default action for the SHUTDOWN menu option on Windows Vista is, believe it or not, SLEEP. Sleep makes the laptop boot up quicker when you turn it on, and that's what Microsoft says people want. The side effect is that Vista continues to draw a minimal amount of power from the battery while it is in Sleep mode. If you don't use your laptop very often, expect the battery to be dead or very low on power when you turn it on days later.
I first noticed this on our home laptop and have confirmed it in at least one of our test laptops at work.
Here's an explanation and here's a step-by-step guide to changing them, if you're not familiar with where they're located.