What you need is the Microsoft Office product key which you obviously don't have, so why don't you try freeware office alternatives?.. there are several options available but to me, nothing compares to the Kingsoft Office Suite which offers a word processor, a spreadsheet program and a presentation maker, and all are readily compatible with Microsoft Office documents, so you can open documents created in Microsoft with Kingsoft, and vice versa.
If this doesn't work, they also offer something called Recover Keys.
Sorry, but that will find the Windows product key... That is what the OP said but probably meant the Microsoft Office product key, which can't be found if Office is not already installed.