All except for the version with the animated clouds background are good for widescreen. Yes, there really is nothing I can do about the animated one. It seems to be a huge system resource drain no matter how its configured.
-Install- First, make sure that the screensaver file is somewhere you're prepared to keep it. After you've installed it, the file is actively used, Windows doesn't copy it over or anything, so put it in a dedicated folder or something.
To install, simply right click on your chosen screensaver and hit 'Install'. A good way to test them out is to right click and hit 'Test'. They are compatible with anything higher than Windows XP.
-Uninstall- Simply delete it. If it doesn't allow you, choose a different screensaver and try again, if still no, try once you restart the computer.
-Credits- Andrew Hussie. Rafira, for testing. Hi-five! Me. I guess. A little.
Given I don't have access to a Mac, it's a pipedream at best.