I don't have a screenshot available for PowerPoint Viewer. But it should be relatively simple to change/add a file association. If you have a PowerPoint file on your computer you can right click and select 'Open With'. Select PowerPoint Viewer if it's among the listed programs. Or, click the Browse button, find PowerPoint Viewer and select it manually. There should also be a check box that says to open all such files with that program by default.
Or, you can also right click a PowerPoint file, then click Properties and select the General Tab. You should see "Open with" and a Change button. This will also allow you to set an association for a new file type.
You may have to first locate where the PowerPoint Viewer is located. If you go to your start menu > all programs, right click PowerPoint Viewer and then click Properties. On the General tab you should see Location:
with the file path for the viewer. It will probably be C:\Program Files\Microsoft\ .... or something similar. This will be to where you have to Browse when setting the file assciation.
Sorry if this is confusing. I sometimes confuse myself.
But here are some screenshots with a PDF file as an example. Using the Open With option and browsing and also the right click option and browsing.
Please tell me I'm not the cause of you pulling your hair out!