The person is disabled, so getting to the PC just to turn it off or hibernate it is quite a hassle. Also, it needs to be able to run Windows XP or greater in order to run her software. One option I'm considering is a Mac mini running Windows via Virtual Box or VMWare or Parallels, but that seems like overkill. Seems like there should be a small quiet reasonably-fast Windows box for under $500 from someone (she already has a nice monitor, and her special keyboard and mouse). Thanks again.
So get a used Mac Mini for probably $300. Any 2009 and up models are still fairly powerful machines, and are better and quieter than any desktop PC I've owned.