Microsoft introduced User Account Control (UAC) in Windows Vista. UAC enables users to perform common tasks as non-administrators, called standard users in Windows (Vista and Windows 7), and as administrators without having to switch users, log off, or use Run As. A standard user account is synonymous with a user account in Windows XP. User accounts that are members of the local Administrators group will run most applications as a standard user. By separating user and administrator functions while enabling productivity, UAC is an important enhancement for Windows.

Disable UAC from the command line

%systemroot%System32cmd.exe /k %windir%System32reg.exe ADD HKLMSOFTWAREMicrosoftWindowsCurrentVersionPoliciesSystem /v EnableLUA /t REG_DWORD /d 0 /f

Enable UAC from the command line

%systemroot%System32cmd.exe /k %windir%System32reg.exe ADD HKLMSOFTWAREMicrosoftWindowsCurrentVersionPoliciesSystem /v EnableLUA /t REG_DWORD /d 1 /f

A couple caveats

1. Must be run with administrative rights.  You could right-click on Command Prompt shortcut and select Run as Administrator.
2. You may have to reboot for changes to take affect.

You can also enable or disable UAC from the Control Panel.