Insert correct DVD-ROM error message on Steam bought game?

[liamshaw95]

I recently bought Cod4: Modern Warfare on Steam, and the multiplayer works fine but everytime i go to launch singleplayer, I get the message Insert the correct DVD-ROm and restart the application message? Is there anyway to fix this as it was a Steam download so i dont have the disc for the game

[DirtyFishy]

http://steamcommunity.com/discussions/#games