I answered a similar question about RAM usage in Mac OS X but the same overall concept can be applied to most any modern version of Windows or any other modern OS.
This all boils down to one thing: There is 100% nothing wrong with your system. You do not need to do anything. You do not need to purchase more RAM. What is happening is the OS is caching programs, processes and data in RAM as part of it’s normal day-to-day operations.
This is done to actually speed up your system if the system needs that data again for some reason. By caching the data the system has used, the system has quicker access to it if it’s needed in the near future. And since this is cached data, this is the kind of stuff that is easily released/freed/tossed/discarded when a more urgent process requires more RAM.
So—for example—let’s say you restarted your system now, logged in and just walked away for a few hours. RAM usage from reboot to the next time you sit down would seemingly inflate, but there is no memory leak happening; it’s just the system caching what little processes it’s paying attention to.
And this happens regardless of how much RAM you have installed. Double or triple your RAM, the caching process will still fill that space up to about the 60-75% mark to take advantage of the amount of RAM installed. As explained in this other answer; bold emphasis is mine:
However, note that most modern operating systems use more memory for
caching. In particular, Vista and Windows 7 are really aggressive with
this. This article does a good job of explaining how memory
management in Vista works; basically, it tries to use up your free
(idle) memory for its cache, and when needed by another program, that
memory is released again.
So relax and don’t worry. Your system is not being slowed down by the RAM usage you are seeing. If anything it’s attempting to speed things up and make your life easier.