Bil Gates changed computers by creating Windows. Before windows was made, you had to type everything you wanted the computer to do. If you want to open a file right now, you just need to double-click on it. Before, you should know the location of the file AND the command t open it. That system was called DOS ( in your computer, you should have MS-DOS, wich is a Microsoft version of the first DOS). Nowadays, DOS is no longer used as a standalone operating system.
The difference between Windows and DOS, its mainly the interface. In DOS you type the commands, in Windows you have your mouse! Simple, but. user-friendly