Skinning a WPF application is as simple as adding an assembly reference (PresentationFramework.Aero) and xml config change. See below.

<Application.Resources>
<Application.Resources>
  <ResourceDictionary>
    <!-- -->
    <ResourceDictionary.MergedDictionaries>
      <ResourceDictionary Source="/PresentationFramework.Aero;component/themes/Aero.NormalColor.xaml"/>
    </ResourceDictionary.MergedDictionaries>
  </ResourceDictionary>
</Application.Resources>
</Application.Resources>

Other sources:

<ResourceDictionary Source="/PresentationFramework.Aero;component/themes/Aero.NormalColor.xaml"/>
<ResourceDictionary Source="/PresentationFramework.Classic;component/themes/Classic.xaml"/>
<ResourceDictionary Source="/PresentationFramework.Royale;component/themes/Royale.NormalColor.xaml"/>
<ResourceDictionary Source="/PresentationFramework.Luna.Homestead;component/themes/Luna.Homestead.xaml"/>
<ResourceDictionary Source="/PresentationFramework.Luna.Metallic;component/themes/Luna.Metallic.xaml"/>
<ResourceDictionary Source="/PresentationFramework.Zune;component/themes/Zune.NormalColor.xaml"/>

References
StackOverflow, http://stackoverflow.com/questions/2075720/windows-7-theme-for-wpf