Information cube bugs

Is an iso downloaded via p2p compatible with steam? - how do i find out my game serial steam

That is the situation. I have an ISO of Half-Life 2: I downloaded uTorrent. I have a Steam account has been created today, in case I need to make the ISO Game.The not yet finished downloading yet so I have mounted with Daemon Tools to see autoplay when the leads me to start the installation. exe. or something else. My question is: if I did not purchase the game through steam, it must be even to play the game, and if so, do I have a number at all? If anyone been in this situation, I would be pleased to tell me what to do. I have never used steam before, so I do not know how everything works. And knowing all the pointer finger, I bought Half-Life 2 in the store when he went, but I lost it somewhere in the hOuse and never recovered. The principle that I can not find the label with the serial number. I have already paid the valve of your money and wanted to spend $ 45 on not to buy. If someone could explain how it works?