Windows Vista, as most know, is the biggest user of memory compared to any other OS in the past. The answer is actually fairly simple: Windows Vista uses most of the system memory as cache via "SuperFetch." This, in turn, allows Vista use as much memory as it can for what it thinks your going to do next. Most people will notice immediatly with Vista that it is one of the fastest running operating systems available, and it is because of "SuperFetch" that it is like this. The down side to this, as you would expect, is that it takes a while for applications to load, because "SuperFetch" needs to move the cache to the hard drive to make room for your application. A more thorough explaination and source of this article can be found here (article by Jeff Atwood of CodingHorror.com).
Windows Vista has a radically different approach to memory management.
Here's Microsoft's explaination of "SuperFetch" (found here and here):
Windows SuperFetch enables programs and files to load much faster than they would on Windows XP–based PCs.
When you're not actively using your computer, background tasks—including automatic backup programs and antivirus scans—run when they will least disturb you. These background tasks can take up system memory space that your programs had been using. On Windows XP–based PCs, this can slow progress to a crawl when you attempt to resume work.
SuperFetch monitors which applications you use the most and preloads these into your system memory so they'll be ready when you need them. Windows Vista also runs background programs, like disk defragmenting and Windows Defender, at low priority so that they can do their job but your work always comes first.