Screen Resolution Problem In Windows 7

Hello,
I have dell studio 1555 laptop. The lcd screen is 1366x768. When i run any game with a resolution of 1024x768 or lower, the game is displayed is only in the middle part of the screen i.e. it doesn't occupy full area of screen and some area from both left and right is black. so plzz tell me how to adjust the 1024x768 and lower resolution on the whole screen.