Sigh.
It is called SuperFetch. Windows 7 learns the kinds of applications you use most often and preloads them into RAM. So, in the likely event you need one of those said applications it will load much faster than if it had to go find the data from your slow-arse HDD.
You pay lots of money for lots of RAM, why are you complaining about Windows making use of it to make your system faster and more responsive? Give it a while, on my last system 7 would use all 4GB and I never suffered any ill effects. This is because Windows will freely give up any RAM to a currently running application as and when it needs it.
When you boot up Crysis, it will clear all the space it needs to run.
In your Task Manager click the Performance tab and then the Resource Monitor. Now click the Memory tab. This coloured bar will show you what Windows is up to.
The green section is the amount being used by
running applications - i.e, stuff you have explicitly opened such as your browser and also background processes. The dark blue section is Standby, this is the stuff Windows has decided to put into RAM just in case. If you open a program with the monitor open, you will see the green section get larger, and the blue smaller. This is Windows giving up RAM for the new application [or if that application was already loaded in RAM, changing state from Standby to In Use.].
In short, it is a good thing. Stop moaning!