Try reinstalling iTunes. If that doesn't work, then contact Apple support. Also, make sure that the iPhone is turned on and you are on the home screen when you are connecting your iPhone.
You just need to go to the Apple website to download iTunes to your computer. Google "download iTunes for Windows" and it'll bring up the page you need.
Go to settings--> General--> Software Update. The phone will automatically detect if there's an available update, and will display it in this menu. Press the link on the bottom of...
You need to unlock your iPhone before you can connect it to your computer. This is a security measure to stop people from accessing your content without permission.