A clean install of Windows 7 using upgrade media

Open regedit.exe

Mossy on over to HKEY_LOCAL_MACHINE/Software/Microsoft/Windows/CurrentVersion/Setup/OOBE/

Change MediaBootInstall from "1" to "0".

Close RegEdit.

Start up cmd.exe with administrator settings.

Enter the following: slmgr /rearm

Hit enter, wait for the confirmation, exit.

Restart computer.

Activate windows!