XnView does not need to be installed on a computer for running it. With some few files on a disk or memory stick it can be run, already.
Please read in topic 'Minimal installation (Windows)' for more details.
This also allows for creating a CD with XnView and images or a slideshow on it which runs on any computer (with Windows installed). The chapter 'Using XnView > Self Starting Slide Shows on CD' in the XnView Online help contains also interesting infos.