This issue is widely discussed so I won't duplicate all the great answers but add my thought about it.
-> The real problem is that those that sold you this thing didn't bother to tell you this would happen. The why and more is out there for you to pick the answer you like. Here's your search:
My dell machine has 4gb of RAM installed but when i right click my computer it displays 3.23GB of ram
Why its showing less ram?