Thank you for the help. I found that an app called intellimemory (by condusiv technologies) was using RAM as cache memory for opening apps faster... removed it and now the usage is around 16-17% with no apps running :)
I am using windows 8 on my laptop (samsung np550p5c-s03)
The task manager shows memory usage around 70% (5.6 GB of 7.9 GB) even when I have no apps running, please help