I bought a new keyboard which came with this software. The EULA says that I am agreeing to auto-updates and that Microsoft can install "updates and/or supplements". I don't want to agree, but the alternative is to return the keyboard (which is logistically difficult, and I need a new keyboard...). My question is, does "supplements" have any particular defined meaning or usage, or would this be giving MS permission to put whatever it likes on my computer?

The auto updates will come with the windwos updates and you still can choose what you want to download anyway. There is no defined meaning for supplements I would say..