The feature Nemo is referring to is called SuperFetch; it caches all your free memory. The purpose of this is to have your most frequently-used apps preloaded into memory to launch faster. It monitors your app-using habits and tracks what days and even what times of day you tend to open certain applications, and makes a guess as to what to put in this cache. It tends to work pretty well, and Vista will liquidate this cache if necessary to make room for other apps. I also have 3GB and it also has cached 2.2GB when idle. You may notice that your hard drive activity light is blinking after booting up for a while; if you look at the Performance tab of the Task Manager, you'll see the "Cached" value in Physical Memory slowly increase as it fills up.
Now if you want, you can actually disable this feature. Hit Start, type "services" and hit enter. Look for the entry titled "Superfetch" and right click > Properties. Under "Startup type:" select Disabled.
