Yes as when you install the game from the CD it will be in the steam program i actually use steam myself and i have bought "Just Cause 2" on CD and the achievements work on that. so all in all Yes! :D
If you buy it off steam for 5 dollars USD, then yes, it is.
You can buy it from Steam or install it with the CD.
if you have the CD key, you don't need a replacement because the game doesn't run from the disk. You can redownload it from Steam instead.
Steam sends games electronically therefore you will need to download it. Most games come with the CD key embedded. The only game I have found to still need the CD key was ETQW (Enemy Territory: Quake Wars). But steam overlay gives the CD key to you when you open it and it asks for it. Hope this helps my steam account is Moo243 if anyone wants to add me.
Either download Steam, and buy the game and download it through the Steam downloader, after that, it will be on your desktop and in your library. Or, you can buy the CD version, for xbox or pc, and manually insert the PC version into your PC or laptop and download it that way.
You can try by adding the game to the steam by clicking the top left conner and selecting ADD program but CD has its own code for files and the online server that steam provides for virtual disks is defferent but you can try it by adding to library hope that helps.
If you are trying to generate the CD key for the game to be played and installed you will have to find the cd key online or you will have to get an actual new game. best way to do it is download the steam client and buy it from them. It will download disk free game for you and you can play it without cd key but like i have said you have to buy it from them its only like 12$
there probably is, but these ways are illegal (except if you buy it on steam or something) and will not be discussed here.
You will need to buy a new cd of mw3 for that. One copy of the game for each account.
You can't.
This is possible, but only if the game already is available to buy from the Steam store. For example, if you have Half Life 1 from back when it was not on Steam, you can validate the CD key on Steam...and then play it without the disc through that. Games that Steam doesn't support however can not be translated over to Steam. Quite a few games are on Steam though, including EA recently adding a number of their games to it.
You can buy her CD at Walmart, Best Buy, or from Amazon.com