It is rather a matter of perspective.
In XP vs. Vista, Vista will automatically pre-cache commonly used applications into memory to improve loading times giving near instant access to things like Internet Explorer, Windows Mail and the like.
Or, atleast that is what Microsoft has to say about why Vista takes so many more resources than XP. In their eyes, the resources issue is a misrepresentation of the apps being pre-cached and then cleared out when other programs need the pre-cached space.
In truth, some apps can load faster on the same hardware after a few tries. Boot time will generally be longer on the same hardware and many resource intensive apps will take longer to load.