The default templates of the WPF controls look different on different versions of Windows.
If you add a reference to PresentationFramework.Aero.dll and set its Copy Local property to true in Visual Studio, you can apply a Windows 7 theme your application by adding a merged ResourceDictionary into your App.xaml:
<Application ...>
    <Application.Resources>
        <ResourceDictionary>
            <ResourceDictionary.MergedDictionaries>
                <ResourceDictionary Source="/PresentationFramework.Aero;component/themes/Aero.NormalColor.xaml" />
            </ResourceDictionary.MergedDictionaries>
        </ResourceDictionary>
    </Application.Resources>
</Application>
Besides Aero, there are some other themes (and their corresponding assemblies) available as well:
<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"/>