Ok. All Dell's ship with OEM operating Systems. When Dell builds your piece of crap and installs the OS on your POC they authenticate the OS online with microsoft. Which is for the DELL PC. During this process information about this POC is sent to microsoft with the serial key for the OS. Now, this OS is only good for 1 machine. Sure you can reinstall it over and over and over again on the same machine, but it's already been authenticated.
Now, try installing this DELL OEM software on a rebuild with a new MOBO and other hardware. You will have to try to reauthenticate it online. And guess what will happen when you try that???
You'll get a message like this one, "This software is already licensed to another computer." Sorry you are SOL!!!!
That's why you need a new OS. Either new OEM or FULL version...