First of all I must say, I don't possess any magic keygen or anyway to get these all for free. If you bought the game, then you should have a key, right? Exactly.
This is meant for those poor souls who purchased and downloaded
Call of Duty 4: Modern Warfare from steam directly, and do not have keys.
Now, without further a due, let's get this started.
STEP 1
Head on over to your start menu. From there open up run.
If you don't see run, then simply click, "All programs", and go to,"Accessories", and open run.
Start>Run / Start>All Programs> Accessories>Run
STEP 2
Whilst you are still in run, clear out any old commands in there, and type in:
regedit.exe
...and press enter. Allow it to be opened, of course!
Run>regedit.exe
STEP 3
Now when you are in
regedit.exe, or shall we call it your, "Registry Editor", double click the
HKEY_LOCAL_MACHINE folder. In here open up the
SOFTWARE folder, and then the
ACTIVISION folder. Finally, open the
Call of duty 4 folder.
regedit.exe>HKEY_LOCAL_MACHINE>SOFTWARE>ACTIVISION >Call of duty 4
STEP 4
BOOM! now you see a bunch of different files! Which one to choose? The answer:
codkey
this is your file which contains your key, obviously. It is indeed a REG_SZ type registration, and the numbers under the data column resemble your key. If you double click it, then you should be able to copy it. Save it to a different document, just incase

.
Note: This is a 20-digit number. When you enter it in-game, it will be divided in to 5 parts, each containing 4 numbers. Confused? Don't be. Just type it in.
Call of duty 4>codkey> YOUR KEY IS HERE!
And there you go!
I apologize if this has already been posted in another thread... I'm just trying to help /
. If this doesn't work for you then... contact steam support and ask for a new key 
I'd also like to be stickied! 