"DOS: No serial number required.
95/98/SE: To cut down on casual piracy, enter this serial number.
Win2K: Since that didn't work, it might phone home unless you ask nicely that it not phone home.
XP: Since that didn't work, it won't activate until you let it phone home. Don't worry, we won't nuke existing installations.
Vista: Since that didn't work, we'll nuke any box that stops phoning."