Microsoft is one of the few companies that attempt to preserve backwards ABI and API compatibility across each version of Windows. For the most part, the majority of the applications out there today that ran under DOS-based operating systems like Windows 9x and Windows ME will run as-is in Compatibility Mode under new versions of Windows. However, there are some applications that require an actual DOS-based operating system to run, and as such, you might come across the need to dual-boot Windows XP/Vista/7 and a legacy DOS-based OS.
The instructions on this page will guide you through the procedure of installing DOS to a system with Windows Vista already installed. These instructions can be used to install any Windows OS from Windows 95 to Windows ME with some minor changes along the way.