Dell inspiron 15 series RAM process over 70% and system running slow

I got a old dell inspiron 15 series from a friend. I installed windows 7 ultimate. It was running over 2GB RAM.  After 2, 3 days the laptop started to run very slow. I checked through the gadgets that the RAM process was over 90%. So i thought that upgrading the RAM would solve my issue. But after upgrading the RAM to 6GB, it still shows the process is over 90%, when i opened google chrome. When the laptop is in idle position, still RAM is over 70%. Dont understand why? What i figured is that the laptop has windows 8 logo in the keyboard which means the default OS should be 8 or above. As Windows 7 is more convenient to use i installed 7 in the lap. I am not sure if that could be the problem. Somebody please help.