How-to and content for a custom Windows screensaver and desktop background
You have a PPT with slides. Now, Do File > Save As and Choose JPEG - instead of Powerpoint Presentation (I guess PNG may work too. Never tried). In the next dialog, say Yes to "All Slides".
Now, ensure these exported slides are in an acceptable location. You may want to delete the title slide image file unless you're okay with that one occupying your desktop background every so often :)
Then, right-click on the (Windows) desktop background and choose Personalize. Next, for Background, change from "Picture" to "Slideshow" and, "Choose albums for your slideshow", click on Browse and selet the Folder you exported the slides to.
You may want to change the setting in "Change picture every" from the default of 30 minutes.