This question comes up a lot when my friends and co-workers are arguing over the merits of Windows Vista. They say, "My {insert some Linux distro here} only uses {insert another useless amount of RAM here} RAM, why is Vista such a hog?" I usually go on to explain to them about SuperFetch and ReadyBoost, and all the advances and better memory management that went in to these two technologies. All information that is freely available from Part 1 and Part 2 of the Inside the Windows Vista Kernel articles.
However, I believe, Jeff Atwood from Coding Horror does a much better job.