Many years ago (before I bought a gaming PC) I bought Assassin's Creed 1 on disc, without realizing my (then) current PC would have no hope of playing it. After discovering this I left the game in a cupboard and promptly forgot about it. Today I was searching through the cupboard and found the game again. Since I now have a decent PC I would like to install and play the game. However, now that I have Steam too, I'd like to install and run the game through Steam. Is this possible? Remember that I have the disc only, no Steam product codes or anything like that.
Unfortunately, you can not activate it on Steam. Here's a list of the games that can be activated with a retail CD Key. However, if you hadn't used the key before you could be able to activate it on Uplay if you do want to use a digital version of the game.
You can install Assassin's Creed from your disc and add a shortcut to the game through Steam using a non-steam game shortcut. However, the game itself will not be tied to your account and you will not be able to download or install the game via Steam. The only way you can tie the game to your steam account is if you purchase the game through Steam's store or enter a Steam redemption key.