Here is an example on how to use the Runas from the command line.
The example will install Open Office on a window 7 computer from the DOS window (aka the black window)

Open the dos window.

Using a Domain account:
runas /user:domain\myadmin "c:\Users\johnsmith\desktop\Apache_OpenOffice_4.1.1_Win_x86_install_en-US.exe"

Using a Local account:
runas /user:ComputerName\myadmin "c:\Users\johnsmith\desktop\Apache_OpenOffice_4.1.1_Win_x86_install_en-US.exe"