One of the hot new features in Windows 7 is the improved window management functions. You can simply drag a window to the top or sides of the screen, and it will be maximized or resized to take up only half of the screen. The only problem is that some people like to drag windows partially off the screen, but in Windows 7 it will resize the window to fill half the screen.
To disable this feature, click on the Start button and select 'Control Panel'. Next, click on the 'Ease of Access' link, then click on the 'Change how your mouse works' link. Check the 'Prevent windows from being automatically arranged when moved to the edge of the screen' checkbox then click on the 'Apply' button.
If You decide later you want to use Aero Snap, simple follow the steps above and uncheck the 'Prevent windows from being automatically arranged when moved to the edge of the screen' checkbox and click 'Apply'.