If you have the game installed your going to have to do a lot of searching for a cd key grabber. Which will let you view your cd key. The only place I found one was on a site for cheating. So be careful on your search scan everything you download, those sites have a lot of viruses.
I did a little looking and found this, I scanned it with avira and it looks clean. I don't have d2 installed at the moment so I do not know if it will work.