this info is supposed to let you use a upgrade disk to install windows 7 on a clean system without a previous OS installed without having to do the upgrade trick where you install the same version using the disk without putting in the serial number then run a upgrade to the same version and put in the serial number trick that Vista introduced.
here is the info, credit goes to whoever originally figured this out. I haven't had a chance to try this yet.
First, perform a normal clean install of the OS by booting the PC with the Upgrade Setup disc and stepping through Setup.
After performing the clean install, ensure that there are no Windows Updates pending that would require a system reboot. (You'll see an orange shield icon next to Shutdown in the Start Menu if this is the case).
Then, open regedit.exe with Start Menu Search and navigate to:
Change MediaBootInstall from "1" to "0" if it isn’t already set to “0”
Change RetailInstall from “1” to “0”
UPDATE: added Change RetailInstall from “1” to “0”
(found this when I actually got around to trying it, used to only have to do the MediaBootInstall entry for vista, this has changed for windows 7)
Open the Start Menu again and type cmd to display a shortcut to the Command Line utility. Right-click this shortcut and choose "Run as administrator." Handle the UAC prompt.
In the command line window, type: slmgr /rearm
Then tap ENTER, close the command line window and reboot. When Windows 7 reboots, run the Activate Windows utility, type in your product key and activate windows. It should just work.