1) Uninstall Combat Arms, uninstall it properly, don't just delete the files.
2) Visit combatarms.nexon.net <-- copy and past that into your web browser.
3) Click on the green button at the top right corner that says "new? click here"
4) It should drag down and if you look left theres a green button saying "Download"
5) Click that and it should load a new page, then click on download full client.
6) Wait for it to finish downloading the downloader, then open it up and let it download the real CA.
Note: This is off my memory so correct me if i'm wrong.