Enable or Disable UAC From the Windows Command Line

C:\Windows\System32\cmd.exe /k %windir%\System32\reg.exe ADD HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Policies\System /v EnableLUA /t REG_DWORD /d 0 /f

Enable UAC

C:\Windows\System32\cmd.exe /k %windir%\System32\reg.exe ADD HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Policies\System /v EnableLUA /t REG_DWORD /d 1 /f

After you enable or disable UAC, you will have to reboot your computer for the changes to take effect.

ref:http://www.howtogeek.com/howto/windows-vista/enable-or-disable-uac-from-the-windows-vista-command-line/