If you use or have used the CoD4 client modification called CoD4X (CoD4 "1.8" and higher) we recommend in the strongest possible terms completely deleting your CoD4 install and re-installing from a clean source such as Steam (a note for downloading from Steam).
The CoD4X client is a privately made, closed-source client which is forced onto your computer when you connect to any CoD4 server running the server version "1.8" and higher. If you're prompted to "Auto-Update" it means you've already been compromised. While there are a lot of pros to the client (Longer client names, backup master server lists, better anti-cheat features, better client ID's and more), it's dangerously invasive. It gives the creator full access to your computer, your files, programs, all of it.
They've spread their modified client via unwitting servers like malware, putting themselves in a position of immense power. After their software reached a saturation point they were happy with, they started forcing updates. Now they even filter out servers not running their software from showing in the server-list of their modified client.
We could go on, but that's the gist. Good concept, with very shady execution. While we can only recommend that you don't use it, a good rule of thumb is to treat your computer like your butt, never give strangers backdoor access.