Were you provided with a COA sticker for the preinstalled Windows 7 as is required to sell it?
If so you can try the Product Key to see if it will activate as it may in fact be genuine. The procedure if it won't is to call MS Customer Service to validate the key. If it is valid they should open a case to get it installed one way or another.
If not you can use a new key. If it won't activate you may need to make the robocall to trade a series of numbers to record the hardware signature in MS Activation Servers.
I make this suggestion so you will know you did everything you can to use the preinstalled Windows 7 license you paid for, and if it works you can use the new license elsewhere.