I am using 3gb but it is showing 2 7 gb




The odd 256Mb is probably on permanent loan to your graphics system. When you first fire up and see a black screen before Windows loads, can you see the figure 256Mb in the top left hand corner?
 
Well first of all... is your graphics adapter on the motherboard or is it an add-on card? If it's onboard, then chances are Saga is correct... the 256MB would be in use by your onboard video card. If you have only an add-on card, then where that memory is going is a bit less clear.