1 Config

Program Files\Call of Duty 4 - Modern Warfare\players\profiles\name

config_mp.cfg - Open it with notepad/wordpad delete everything, now open the downloaded config and copy everything in it, to your config_mp.cfg. Now save it.

Multiple Configs

Program Files\Call of Duty 4 - Modern Warfare\players\profiles\name

Place the downloaded config in there, then in game run this command in the console

/exec configname.cfg

That way you have have multiple config files and swap between them in game