I'm not sure there is anything wrong with your computer, other than it's getting a bit old and out of date. 60% to 70% of ram under normal usage is not a problem. Windows 10 or any other OS is going to use ram based on how much is available for it to use. If you only had 4gb of ram Windows 10 would adjust to that.
As for your games dropping to 15fps, that all depends on what games you are playing. Some games require a faster video card to run smoothly, and a GTS 450 is becoming relatively slow by today's standards.