Would you like your machine to use up to 25% less power? Are you on XP now? You can see a big improvement by switching to Windows 7. There are some other things you can do too, like changing some drivers, but those are probably a bit difficult for most people. We have a number of machines kicking around the office that we've left on XP because they're really just file and print servers, people don't use them directly very often, so the obvious UI benefits of Windows 7 didn't seem relevant. But lowering the power bill is relevant, right? Or, if you're out and about with your laptop, having the battery last longer is relevant, right?
Here's the blog
where I found some numbers ... it's a summary of a longer whitepaper
and you should probably read that too.