What the fake windows 7 update screen is
Before the flat blue era, Windows applied updates on a plain black screen with white text and no spinner at all — just a number and a warning. It looked far more alarming than the modern version, which is exactly why it works as a prank.
Windows 7 reached end of support in 2020, so a machine genuinely showing this screen today is a story in itself. That works in your favour: the reaction is usually confusion first, then suspicion.
Windows 7 update prank, no install
Press F11 first. A black screen with white text is the easiest of all of these to sell, because there is no interface for the browser chrome to clash with.
The percentage stalls at random. On a real Windows 7 update it could sit at 35% for a genuinely unreasonable length of time.
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.