Another way to activate a OEM copy of windows on a Dell computer

This solution was provided by Anonymousnone on Technet

Copy the install.wim from the Dell cd to your computer's hard drive
xcopy D:\sources\install.wim c:\WinPE\install.wim

Identify the index number of the Windows 7 pro install
dism /Get-ImageInfo /imagefile:C:\WinPE\install.wim

Extract the Windows 7 pro install from the Wim file
dism /export-image /sourceimagefile:install.wim /sourceindex:3 /destinationimagefile:DellPro7.wim

Mount the extracted WIM
dism /Mount-Image /ImageFile:c:\WinPE\DellPro7.wim /index:1 /MountDir:c:\winpe\dell7\

Copy the files from the Dell DVD to the mounted WIM file structure
xcopy D:\sources\$OEM$\$$\setup\ C:\Windows\Setup\ /E /H
xcopy D:\sources\$OEM$\$$\system32\ C:\Windows\system32\ /E /H

Remount the image commiting the changes
dism /unmount-image /mountdir:c:\winpe\dell7 /commit

Import the newly WIM file into MDT 2012 and add it to your OS deployment task or import into Windows deployment services.