There is no conventional Start Menu in Windows 8 like to that of in Windows 7 which shows programs and files while typing on the the search box, if you want run command as administrator in Windows 8 here is how.
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
3, cmd appears on the left pane, right-click on it click on Advanced at the bottom and select 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 C Drive
Now from File menu select Open Command Prompt>Open Command Prompt as Administrator