You can get an analysis of how your Windows operating system's RAM is being used by downloading and installing RAMMap, a free program available here.

As you can see it breaks down how much RAM is being used by the Kernel (the core of the operating system, which loads first during start up and controls the rest of the system); Mapped Files (files which are assigned memory byte for byte in order to increase performance); Page Table (used to map the virtual address that accessing processes use to get the actual physical address of data - which can be dispersed); Process Private (memory that isn't shared amongst processes); and Paged & NonPage Pool (pool data is kept ready to use, rather than being acquired when it's requested, - paged data may come from a secondary source.)

If you click on the Priority Summary you can see how much memory pressure your system in under. If there is a lot of re-purposed memory with a priority between 5 and 7, there will be significant memory pressure on the system. Memory pressure is a better way of measuring how muchstress your system is under than how much free RAM is available, since keeping more than a few MB of RAM is wasteful.