See intro blogpost here.

The namespaces in WinRT has changed for all the UI related classes. Generally System.Windows has been replaced with Windows.UI.Xaml, and the rest is the same below that. If we to use the “NETFX_CORE” compiler directive that WinRT projects comes with, the typical default using statements that would compile on both Silverlight, WPF and WinRT would look like this:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
#if !NETFX_CORE
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;
#else
using Windows.UI.Xaml;
using Windows.UI.Xaml.Controls;
using Windows.UI.Xaml.Data;
using Windows.UI.Xaml.Documents;
using Windows.UI.Xaml.Input;
using Windows.UI.Xaml.Media;
using Windows.UI.Xaml.Media.Imaging;
using Windows.UI.Xaml.Navigation;
using Windows.UI.Xaml.Shapes;
#endif

This is also described in greater detail here: http://msdn.microsoft.com/en-us/library/windows/apps/hh465136(v=VS.85).aspx