Firstly, as Murtuza says, check your Internet connection. If you can connect to Internet without issues, configure your security program and Windows Firewall to allow iTunes to communicate. If the problem persists, remove iTunes from your computer (Control Panel > Uninstall a program), run a CCleaner or Glary Utilities and download and install the latest version of the program from the official web page. Also, while downloading the attachment, close any unnecessary application that uses your Internet bandwidth.