If word won't activate, ring up microsoft and they can check, if it is a bad key they will give you another, if it's been activated or is dodgy, take it back to the shop, cry foul and tell them to expect a call from microsoft legal department and get a refund/compensation.
Are you sure that they had the full version installed? Usually when you get a new laptop it comes with a trial version that expires after 30,60,90 days or something similar. Now if it was the full version and a key was put in already, that's no good, follow das_stig's advice then.
I purchased my computer from Rent-A-Center . I believe that the Microsoft Word Key code that was included on my desktop was used.
So you bought a computer from a place that rents out computers to other people. Unless you bought that computer brand spanking, never used before, new then my friend you have just experience the digital version of Sloppy Seconds!! Enjoy!