Vista - How to disable animation for changes, which made during initialization?

I have the code like this:
.....
<Window.Resources>
....
<ControlTemplate>
<ToggleButton IsChecked="{Binding
RelativeSource={RelativeSource TemplatedParent}, Path=ShowAll}"
x:Name="abc" />
<Image source="abc.bmp" x:Name="def" />
<ControlTemplate.Triggers>
<DataTrigger Binding="{Binding ElementName=abc,
Path=IsChecked}" Value="True">
<DataTrigger.EnterActions>
<BeginStoryboard>
<Storyboard>
<DoubleAnimation Storyboard.TargetName="def"
Storyboard.TargetProperty="Width"
To="500" Duration="0:0:0.5" />
</Storyboard>
</BeginStoryboard>
</DataTrigger.EnterActions>
</DataTrigger Binding="{Binding ElementName=abc,
Path=IsChecked}" Value="True">
</ControlTemplate.Triggers>

</ControlTemplate>
....
</Window.Resources>

When the program is starting, the animation start, if i set property
ShowAll to true. How to make so, that animation will run only when
changes in property ShowAll are makes after initialization?