Instead of setting the opacity of the window, set its background’s opacity:
<Window x:Class="WpfApplication3.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"
AllowsTransparency="True" WindowStyle="None">
<Window.Background>
<SolidColorBrush Opacity="0.5" Color="White"/>
</Window.Background>
<Grid>
<Button Width="200" Height="50">button</Button>
</Grid>
</Window>