The downloader that CO downloads through uses Pando Media Booster, which continues to run in the background even when the download is finished and the game is installed. It tends to hog your bandwidth to obscene levels, and has no configuration options to stop it.
Once your initial download of the client is finished, open up your task manager and kill PMB.exe, then uninstall Pando Media Booster from your Add/Remove Programs window.
The actual launcher for the game, which handles patching the game, does not use Pando Media Booster, and is safe.
The BitTorrent download does not use Pando, and judging from the talk here in the thread, neither does the Steam version. This makes sense, as BitTorrent can easily handle the 3+ gigs of the full client w/o the need for a downloader, as can Steam.