But Windows 1.0 was marketed as an upgrade for people already running MS-DOS — and, in fact, it ran on top of MS-DOS, so anybody who wanted Windows had to have MS-DOS installed first.

Not so long ago, IT admins would scorn the use of an in-place upgrade for Windows hardware. But you don't have to do everything from scratch now.

Technically, upgrading Windows XP to a newer OS is an option, but it doesn’t make any sense from a financial perspective.