What the sarcastic fake update screen is
This is the version for when you want the joke to be found rather than believed. The layout and pacing are identical to a real update screen; only the status messages give it away, and only if someone actually reads them.
That delay is the funny part. People have learned to stop reading update screens, so the line about deleting System32 usually sits there for a while before anyone notices.
Funny fake update screen with changing messages
Messages change every few seconds, so leaving it running gives whoever walks past several chances to catch on.
Nothing is installed and nothing is deleted, including System32. That is a joke, not a threat.
How to run it, and how to stop it
Press F for fullscreen, then F11 to hide the browser’s own toolbars. That second step is the one that matters: an address bar sitting above the screen gives the whole thing away instantly.
To end it: Escape leaves fullscreen, F11 brings the toolbars back, and closing the tab removes it entirely. Nothing is installed, no file is written and no setting is changed, so there is nothing to undo afterwards.
Worth saying plainly — use this on people who will find it funny, and on machines you are allowed to touch. Show them the way out once the joke has landed.