What's New in User Account Control(7,8 r2):

Before the introduction of User Account Control (UAC), when a user was logged on as an administrator, that user was automatically granted full access to all system resources. While running as an administrator enabled a user to install legitimate software, the user could also unintentionally or intentionally install a malicious program. A malicious program installed by an administrator can fully compromise the computer and affect all users.
With the introduction of UAC, the access control model changed to help mitigate the impact of a malicious program. When a user attempts to start an administrator task or service, the User Account Control dialog box asks the user to click either Yes or No before the user's full administrator access token can be used. If the user is not an administrator, the user must provide an administrator's credentials to run the program. Because UAC requires an administrator to approve application installations, unauthorized applications cannot be installed automatically or without the explicit consent of an administrator.
In Windows® 7 and Windows Server® 2008 R2, UAC functionality is improved to:

• Increase the number of tasks that the standard user can perform that do not prompt for administrator approval.
  
• Allow a user with administrator privileges to configure the UAC experience in the Control Panel.
  
• Provide additional local security policies that enable a local administrator to change the behavior of the UAC messages for local administrators in Admin Approval Mode.
  
• Provide additional local security policies that enable a local administrator to change the behavior of the UAC messages for standard users.
  

Who will want to use UAC?

UAC helps standard users and administrators protect their computers by preventing programs that may be malicious from running. The improved user experience makes it easier for users to perform daily tasks while protecting their computers.
UAC helps enterprise administrators protect their network by preventing users from running malicious software.

What are the benefits of the new and changed features?

By default, standard users and administrators access resources and run applications in the security context of standard users. When a user logs on to a computer, the system creates an access token for that user. The access token contains information about the level of access that the user is granted, including specific security identifiers (SIDs) and Windows privileges.
When an administrator logs on, two separate access tokens are created for the user: a standard user access token and an administrator access token. The standard user access token contains the same user-specific information as the administrator access token, but the administrative Windows privileges and SIDs have been removed. The standard user access token is used to start applications that do not perform administrative tasks (standard user applications).
When the user runs applications that perform administrative tasks (administrator applications), the user is prompted to change or "elevate" the security context from a standard user to an administrator, called Admin Approval Mode. In this mode, the administrator must provide approval for applications to run on the secure desktop with administrative privileges. The improvements to UAC in Windows 7 and Windows Server 2008 R2 result in an improved user experience when configuring and troubleshooting your computer.

The built-in Administrator account in Windows Server 2008 R2 does not run in Admin Approval Mode

The built-in Administrator account in Windows Server 2008 R2, which is the first account created on a server, does not run in Admin Approval Mode. All subsequently created administrator accounts in Windows Server 2008 R2 do run in Admin Approval Mode.

The built-in Administrator account is disabled by default in Windows 7

The built-in Administrator account is disabled by default in Windows 7. The built-in Administrator account, by default, cannot log on to the computer in Safe Mode.

Behavior of computers that are not domain members

When there is at least one configured local administrator account, the disabled built-in Administrator account cannot log on in Safe Mode. Instead, any local administrator account can be used to log on. If the last local administrator account is inadvertently demoted, disabled, or deleted, Safe Mode allows the disabled built-in Administrator account to log on for disaster recovery.
If the built-in Administrator account is the only administrator account on Windows Vista, when upgrading to Windows 7, Safe Mode allows the disabled built-in Administrator account to log on to create at least one administrator account.

Behavior of computers that are domain members

The disabled built-in Administrator account in all cases cannot log on in Safe Mode. A user account that is a member of the Domain Admins group can log on to the computer to create a local administrator if none exists.