I'm looking at adding that additional layer of security, however, I find the numbers on the physical authenticator rather difficult to read. Is it possible to use the mobile authenticator on a windows 7 desktop pc?

I don't believe so. The whole point of the device is that it's on a different "machine" than the one you use to play the game. However, if you find that the mobile key fob is hard to read, the security key is available for iPods, iPads, and many phones and such too.