This one is cool: [Wayback/Archive] PRANK: Windows XP Updates.
Note that unlike the screenshot below, the actual prank does count the percentage. The actual page does.
You can start this one and various other OSes plus Windows versions and other pranks via [Wayback/Archive] FakeUpdate.net – Windows Update Prank by fediaFedia (at the time of writing Windows 98 install, Windows Vista update, Windows 8 update, Windows 7 update, Mac OS boot, Windows 10 install, Windows 10 update, steam and “fake ransomware”).
It is a cool and relatively harmless way of teaching people to use their lock screen when away from their machine (Windows: Win
+L
, Mac OS: Ctrl
+Shift
+Power
).