You haven't go it the CD, you downloaded it, now you need to crack it.
Search for a GTA IV crack, then crack the game.
Then you can play.
The reason it says NO-CD, is because it thinks you have a CD to put in your computer.
So crack it, then you're golden.
Note: If you don't know how to crack GTA IV they is many tutorials that you can follow.
You don't really explain your problem heres my questions, above is my assumption that you pirated GTA IV.
Did you buy the game and install it?
Uninstall it, and re-installing it might be your solution.
Or is the disk you have been damaged? Have you downloaded the latest updates what error did you get?
Give some more detail or I and no one else can help you.