There is no conventional Start Menu in Windows 8 like to that of in Windows 7, which shows  programs and files while typing in the search box, if you want run command as administrator in Windows 8, here is how you can do that.

cmd prompt opened with  admin previlages

Most easiest method in Windows 8: Quickly Open Elevated Command Prompt with Win+X, A Shortcut

Running command Prompt with Admin Privileges in Windows 8

1. Press Win key +F for metro search UI to appear

2. Click on Apps and type cmd in the search box under Search for Apps and press enter

apps results for cmd

3.cmd appears in the left pane, right-click on it and click on Advanced at the bottom and select run as administrator

run as administrator

Command prompt window opens running with admin privileges appears as shown on top in the screenshot.

Update 06/01/2012: Even easier method, follow the instructions below

Open Windows Explorer or My Computer ,and Open the C Drive

Now from File menu, select Open Command Prompt Open Command Prompt as Administrator