In Windows, how can I complete a task or run a program as administrator while logged in as a regular user?

Completing a task with administrative rights
In Windows 7 and Vista, when you begin a task requiring administrative privilege (e.g., installing or upgrading software, configuring certain operating system parameters), you will be prompted to enter the username and password for your administrative local account.

Running a program as administrator
In Windows 7 or Vista, if a particular software program on your computer requires you to be logged in as an administrator, right-click it from the Start menu, and select Run as administrator.
In Windows XP, navigate to the program you wish to launch with different privileges. Click it once to select it, and then hold down the Shift key and right-click the program. You'll see a contextual menu from which you can select Run as... and then choose a different account.

In Windows 7 or Vista, from the Start menu, in the "Search programs and files" field (in Windows XP, from the Start menu, select Run...), enter the following command:
  runas /user:username cmd
Replace username with the name of the account you want to use. You can replace cmd with any command you'd like to run (if it contains spaces between characters, enclose it in double quotes). Click OK. When prompted, enter the password for the account you've chosen. This method will start a new command shell in a new session, from which you can launch programs.