I did something like this, and you can work on it.
Unfortunately you have to implement your own Window title bar :
<Window x:Class="WpfApplication2.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="MainWindow" Height="350" Width="525"
Name="myWindow" WindowStyle="None" AllowsTransparency="True"
>
<Window.Resources>
</Window.Resources>
<Grid>
<Button Margin="337,0,0,155">
<Button.Triggers>
<EventTrigger RoutedEvent="Button.Click">
<BeginStoryboard >
<Storyboard Duration="0:0:5">
<DoubleAnimation Storyboard.TargetName="myWindow" Storyboard.TargetProperty="Opacity" From="1" To="0"></DoubleAnimation>
</Storyboard>
</BeginStoryboard>
</EventTrigger>
</Button.Triggers>
</Button>
</Grid>
</Window>