You can transfer Word licences between machines. So if you have an existing licence, you can move it to the spare machine. It is a transfer, so if you do this, it will stop working on the original machine. What version of Word do you have?
If you are not paying close to the list price for the software, then it is almost certainly pirated. Quite often the pirated activation code works initially, but eventually Microsoft notice that they never issued it, and it doesn't work again. Even if you are paying close to the list price, you have to be careful that what you are buying is genuine. I steer clear of eBay and Amazon Marketplace sellers when buying software.
Libre Office is good, and its free, and there is absolutely no need to move to the .ODT format. Just change the default save formats to .dox/.xlsx/.pptx. Its not good if you use macros, or if precision positioning or exact selection of fonts is important to you (but then you're probably not using Word if it is).