1. How many of you have used the IBM Computer with DOS sitting on it?
Never, I am not THAT old!
2. Which was the first Operating System (OS) you have used ?
Windows 98
I don't remember whether my first interaction with computers was in school learning Logo / playing DAVE, OR when I saw a monochrome display with white texts (DOS) on it at my big cousin's room!
But I consider the origin of my Geek life at my Mausi's Pen 3, Win 98 computer, on which I used to play "Carmageddon II: Carpocalypse Now" and do C++ programming later on.
I do remember the Floppy drives, in which I used to carry "files" and used to store it like it was the most precious thing in my life! And I remember the Autorun virus that was a nightmare with those disks! Counting in MB's was A LOT those days.
I also remember the Sundays for which I always used to look forward to, not because it was the holiday. But because, it was "Internet Day" for me. And I used to plan each and every thing the whole week, for whatever I wanted to do in that 1 or 2 hours in the Cyber Cafe! Like, the content which I would write in the mail, and whom to mail (cousins).
Well, this may sound extreme Geek nature, but I had
drawn (by sketchpen) a diagram of a keyboard, and used to "practice" on it, so that I can type faster in front of the actual keyboard in cafe!

Ah.. the good old days, of my childhood!
My first (and the one) "PERSONAL PC" was only when I reached college! That was/is Pen 4 with XP.