xmlns:local="clr-namespace:wpf_test"

<Window x:Class="wpf_test.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:sys="clr-namespace:System;assembly=mscorlib"
    xmlns:local="clr-namespace:wpf_test"
    Title="MainWindow" Height="350" Width="525"
    x:Name="MySlidingWindow">

<Window.Resources>
    <sys:Int32 x:Key="WindowPosition">30</sys:Int32>
    <local:WindowPositionConverter x:Key="WindowPosConv" />
</Window.Resources>

<Window.Triggers>
    <EventTrigger RoutedEvent="Loaded" SourceName="MySlidingWindow">
        <EventTrigger.Actions>
            <BeginStoryboard>
                <Storyboard>
                    <DoubleAnimation
                        Storyboard.TargetName="MySlidingWindow"
                        Storyboard.TargetProperty="Left"
                        From="{x:Static SystemParameters.FullPrimaryScreenWidth}"
                        To="{Binding Source={StaticResource WindowPosition}, Converter={StaticResource WindowPosConv}}"
                        Duration="0:0:1.5"/>
                </Storyboard>
            </BeginStoryboard>
        </EventTrigger.Actions>
    </EventTrigger>
</Window.Triggers>

<Grid>

</Grid>
</Window>

using System;
using System.Globalization;
using System.Windows.Data;

namespace wpf_test
{
    class WindowPositionConverter : IValueConverter
    {
        public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
        {
            var pos = (int) value;
            var result = System.Windows.SystemParameters.PrimaryScreenWidth/100*pos;
            return result;
        }

        public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
        {
            throw new NotImplementedException();
        }
    }
}