Hi 1. Assuming you have Powerpoint. Open the presentation and then save as and scroll to .jpegs and save them in a new folder. You will then have all the slides in a folder that you can open and discard any that you do not want.
2. If you have not got Powerpoint but only the Powerpoint Viewer. You will have to play the presentation and by hitting the "Print Screen" button on the keyboard this will save that slide to the clipboard. In your photo editing Program Open File from Clipboard and then save the slide in a folder. You will have to do this for all the slides.
Having now got all the slides in either method 1 or 2 in a folder right click on the Desktop and then Display Properties then Screen Saver then My Picture Slideshow then Settings then Browse to the folder with the slides .
I think you will find copy and paste does not work for copying slides in a Powerpoint show and Irfan will only create a screensaver from a folder of slides as it cannot open Powerpoint presentations.