Ok here I will show you how to play cod4 with garena because you can actually play online there without an original keycode.
You need the following :Garena Client
:Account for Garena
Call of Duty Modern Warfare(installed in your pc) and then download and install the patches from 1.1 to 1.7(You can simply download it no need to buy it)
Patches:v1.1 Patch
:v1.2 Patch
:v1.3 Patch
:v1.4 Patch
:v.1.5 Patch
:v.1.6 Patch
:v1.7 Patch
When you are ready you can login to garena and select from the game category the COD4 icon
Then you choose a room
After finding a room you need to adjust some settings,when you are into the room tou press this button
Then you select the game icon and you must write the path of your iw3mp.exe After that you click update.
Now you press start game
Now you go to "Join Game" then you change the source to local and the servers of the room you are into will appearhttp://img809.imageshack.us/img809/2783/dwduwgd.png[/img]
Now you can select your server and play!
Have fun.
Credits for the guide:Salot.