See also: Windows 7 Special Report
Probably the most talked about security change in Windows 7, scheduled for public release on Thursday, are modifications to the UAC, which was introduced in Vista. The UAC was designed to prevent unauthorized execution of code by displaying a pop-up warning every time a change was being made to the system, whether by the operating system or a third-party application.
Vista users complained that they were bombarded with the warnings and security experts speculated that as a result, many people were just ignoring them or turning them off.
With Windows 7, users can choose how often they want to be notified and the default is set to notify only when a third-party application is making a change, as well as when a change is being made to the UAC itself.
For more, read "Windows 7 default user account control worries experts" from CNET News.