Look at what memory you have installed. Control Panel/System/General tab.
Look in SystemAdvanced/Performance Settings to see if Windows is managing. In there you can set a fixed size .. I use 1 1/2 x what I have installed as RAM. Custom size min and max same.
You may need to add extra RAM, but this "may" get you out of trouble for the time being.
You "may" have too many background processes running due to installed software running system tray entries, etc..
You should also check that out.