Vista took up a stuff ton of resources.
When people first move from Windows XP to Windows Vista or Windows 7 and bring up a RAM usage meter, they often panic. What they see is something like Figure A.
Figure A
Does Windows 7 really use more than 8 GB of RAM?!
Wow, that looks scary, doesn’t it? The system is doing just about nothing (1% CPU usage) but needs 8.84 GB of physical RAM to run. Here’s what is really happening.
Starting in Vista, Windows got aggressive about RAM use. The engineers at Microsoft made it pre-allocate RAM and pre-cache commonly used items, even if they were not actually in use. For example, if you use Word a lot, it will keep Word in memory ready to be used. Obviously, this lets it chew up a ton of RAM, and why not? It isn’t like you were using the RAM anyway, and you will eventually be using it, most likely for the purposes Windows is preparing for. Applications start much faster as a result.
http://www.techrepublic.com/blog/10things/dont-be-fooled-by-these-10-pc-performance-myths/2703