The Windows screensaver file extension is .src, but they're really just .exe in disguise. You can set any .exe to your screensaver by changing the extension to .src and selecting it under personalization settings.
@RegalBeagle sounds like a great way to set up a ticking time bomb. i have not used a screen saver since windows xp, probably not much even then though. i guess because today it is modern to just hibernate the computer and there is little need to keep it running while you are not there.
@Justicar I don't know for sure this still works, but I remember setting a win95 machine to use the defragger as a screensaver once because the old defragger used to actually display a gui of the data blocks as it ran.
https://dullneon.com/randomnotes/images-videos-and-other-content/2017/04/defrag_working.gif
@RegalBeagle i remember that too :D