I've always had DELL machines and usually there is either the media CD for installation (with older purchase) or a piece of paper which has the Office installation product key on it. Are you able to look and find the physical piece of paper that had the product key?

I know there are software programs available to download which will scan your windows registry and identify any product keys so you could try that but not sure if i would find the old product key if you have now reinstalled over the original.

Unfortunately without having the product key you are not going to be able to activate the suite. I am not aware of any way around it other than having to buy the product again.