A common problem for Windows Phone developers moving to Windows 8 is the support for XNA. XNA is not part of the “Metro” types of apps, but you can still develop XNA using Windows 8 (like you do in Windows 7). Installing the SDK directly in Windows 8 will make the XNA part of the installation to fail.
The solution is very simple. Make sure you have the latest version of Games for Windows Marketplace Client installed.
Follow these steps to install Windows Phone SDK 7.1.1 with XNA on Windows 8:
1. Install Games for Windows Marketplace Client
2. Install Windows Phone 7.1 SDK
3. Install Windows Phone 7.1.1. SDK Update