Do not attempt to do this if you are not comfortable. It is a tricky thing to do and should be only attempted by a very knowledgeable user. If I were you, I would take the laptop to a computer specialist (like Geek Squad) and have them diagnose and correct the problem.
I just noticed that dxdiag show that my GPU have 4Gb memory even thought it have only 2Gb, can that be the cause ?
Edit: If I do a 64bits scan dxdiag show 1751MB memory