The best advice comes from here.
The often culprit is usually the browsers (IE, Google Chrome and Firefox) which are memory hogging esp if you like to open many tabs like me. My own experience is that Google Chrome is the most often culprit that causes this “Your computer is low on memory” error message. In the case of Firefox, it will just crash without this error message being warning displayed. As a rule of thumb, webpages that have lots of embedded videos and pictures like some video playing sites (e.g. Youtube), e-commerce sites (e.g. Qoo10.sg, Ebay) and mail applications (gmail, ymail) are the ones that you don’t want to open too many tabs and leave them there.
By opening the Performance tab of Windows Task Manager (press the Ctrl-Alt-Del keys simultaneously), I notice the Google Chrome (which has 22 tabs opened) uses more than 3.5GB of the RAM!!!