Windows 7 by default comes with the Snap feature enabled which automatically resizes open windows when they are dragged to extreme corners of the screen. This comes in handy when sizing windows quickly or keeping open windows side by side for comparison.
However, in order to turn this feature off :
1. Open Control Panel > Ease Of Access Center.
2. Select Make it easier to focus on tasks.
3. Enable the tick mark next to the option of Prevent windows from being automatically arranged when moved to the edge of the screen.
This will turn the Snap feature off in Windows 7.