Many of my friends have expressed frustration over some of Vista's "features" which make it so power-hungry and slow. Here's an article from Information Week which talks about a program named vLite, which can strip Vista down to a lean and mean operating system. It is used in the installation mode, so unfortunately it is not designed for those who already have Vista running. (You use it during the installation phase: as the makers of the tool say, "vLite is a tool for customizing the Windows Vista installation before actually installing it.") It is excellent in this mode, and besides, it's free!
UPDATE: another journal article from Computerworld praises this tool and indicates that it can reduce the installed size of Vista from 15 gigs down to 1.4 gigs - a very significant reduction in footprint! And, by the way, there's a similar tool for Windows XP from the same programmer, called nLite. It's not nearly as necessary, as XP is not nearly as bloated as Vista, but for those who insist on the most finely tuned system it is worth using.
What about those who already have Vista installed? Here are a few sites and tools. The instructions from the folks at ExtremeTech are not as technically involved and dangerous as some I've seen. Also, check out Mobile Pedia, the instructions at the TweakVista site, and those at cucirca.com. The latter makes references to a tool called Tweak VI, which is available as a free download and will automate a number of the processes described on these other sites, which you would otherwise do manually. This (Tweak VI) is probably your best choice.