This is a nice link about the history of Windows. The earliest I can remember using is Windows 3.1 in third grade... And minesweeper was my favorite game in the world! The feeling after beating the "computer" in guessing the mines was euphoric! I'm one of the few millions in the world who would NOT have studied computers if there was no graphical user interface. I still prefer clicking my way through to actually typing the commands, though the command-typing-green-lines-on-a-serious-looking-black-screen is cooler. Geeks may deride my admission but that is how it is! And I know that there are millions of others who are glad for the simple-to-use GUI. And all said and done, Windows with its GUI paved way for faster integration of computers into the lives of common man all through the world (I'm not talking about geeks who would've been happy typing commands into command line window with or without GUI) and made computers less fearful. The growth of Windows in all areas of computing(UI, power, performance, etc) has been impressive. And, I'm glad about Windows 7. :)
What is the earliest Windows that you have used?