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User Account Control, or UAC, is the latest in Microsoft’s anti-malware weaponry. UAC blocks all user-initiated and malware-initiated actions on a workstation that require administrative privileges.
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The intention of UAC is to make users aware of all privileged operations on the system, including any that are initiated without their knowledge by malware.
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UAC goes a step further. If a non-privileged user wishes to perform a privileged action, such as edit the system registry, UAC will not only alert the user of this fact, but provide a means for the user (or some other person) to enter privileged-user credentials (like a cash register supervisor over-ride) that will permit the privileged operation to take place.
From an upcoming article on Vista UAC
Peter H. Gregory 