|The Product Key that's on the official Microsoft label on the outside of the computer case is for the version of Windows that was originally installed on the computer, if there's only one such label.|
If you have Microsoft Office installed on the hard drive, if you don't know what it's Product Key is, you can use certain third party programs to detect it's Product Key.
E.g. Search for : Keyfinder by Jelly Bean whatever on the web
(It also finds the Product Key Windows is using, which may be different than the one on the label on a brand name system, and the Product Keys of other Microsoft products.)
If you DO NOT have Microsoft Office installed on the hard drive, there is no need these days to buy it.
You can install free readers available from the Office support pages of the Microsoft web site for Word, Excel, PowerPoint, etc. that allow you to read Word, Excel, PowerPoint, etc. files, but you can't make them or edit them with the readers.
OR you could install a freeware alternative to Microsoft Office.
E.g.Open Office can read, make, and edit files Microsoft Office normally makes.