Disable your anti-virus software , firewall and then run the downloader again and wait to see if it'll be ok. It's better not to put the game in your logic drive, (Drive C) because the UAC system will cause some trouble when you run the game.
If the downloader still won't work on your side, i recommend you to get a download manager and download the client with the help of it. Client link, http://down5.enjoymmos.com/MWOSetup1006.exe |