Maybe it is a pirate copy.

Pirates use something called a crack to do it of your tale. The game is installed then with the crack the CD is not needed anymore.

I first encountered Diablo when my neighbor was playing the original, and it just looked like a ton of fun. My brother, who is a year or two older than me, bought or received the game as a gift sometime later that year. I played it a bit and liked it. When Diablo II came out, he got that for Christmas or something and I got so into it. There was a 2-3 year period where I was hooked. Our older sister even got into it for over a year, hardcore, and she’s not the type to play video games (with the exception of Dr. Mario and Harvest Moon). We used to fight over the computer over who got to play Diablo II. I still play it here and there, for several months at a time, every year without fail. My favorite video game. I cannot wait for Diablo III.