Windows 7 features interesting new ways to intelligently arrange your windows, so that (for example) if you drag a window to the top of the screen then it will maximize. If you find these new features distracting then they're easily disabled. Go to the Registry Editor and look for the following string:
In the right pane, double-click on the 'WindowArrangenmentActive' DWORD Value and set the Data Value to '0'. Reboot your computer for the changes to take effect. Your windows will now behave as they used to.