You can use an installation of Microsoft Works to get Microsoft Office at upgrade prices, but there appears to be no qualification to use Works itself.
The only caveat is that certain sellers in the "used and new" section are selling OEM copies at very low prices. The OEM copies do not provide tech support unless you bought a computer from the company named on the CD or DVD case. However, you can (as I have done) installed an OEM copy and used all the software including Microsoft Word without any problems.
Again, given in my review I believe this to be one of the bargains of software, I don't see any reason why this shouldn't be on your computer. I am about to sell my computer and I am buying a new copy for my new computer. I'm sure the person who buys my old computer will think it a bargain that so much software is legally included.