Well first i would call them. Tell them your situation. Tell them your hard drive crashed and that you had to get a new one. Tell them that you are using the serial given to you with the machine. They should then ask for the code given on screen. After you give it they should not give you a hassle and give you the activation code to make it legit.
I mean it is a Dell right? So the serials are legit. It is just that they were used for your first install. That is why it says they are no longer legit. OEM has only 1 internet activation. Sucks i know.