What the fake windows xp update screen is
XP updates ran on a dark blue field with a chunky segmented progress bar rather than a smooth one. The typeface is Tahoma, which is a large part of why anything from that era looks the way it does in memory.
This one lands differently from the modern versions. Nobody believes their laptop has spontaneously reverted to Windows XP, so it reads as a joke almost immediately — which for a lot of people is the better outcome.
Windows XP update prank screen online
Good on an office display or a second monitor where the anachronism is the joke rather than the deception.
Pair it with the Windows XP boot screen for a longer bit: boot, then update, then boot again.
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.