User Account Control (UAC) is a technology and security infrastructure introduced with Microsoft's Windows machines. It aims to improve the security of Microsoft Windows by limiting application software to standard user privileges until an administrator authorizes an increase or elevation. In this way, only applications trusted by the user may receive administrative privileges, and malware should be kept from compromising the operating system.

Explanation
At minimum two-factor authentication requires two out of three regulatory-approved authentication variables such as:
* Something you know (like the PIN on your bank card or email password).
* Something you have (the physical bank card or a authenticator token).
* Something you are (biometrics like your finger print or iris pattern).

Reference: http://en.wikipedia.org/wiki/HTTP_cookie