Fixing a bug can cause bugs
Microsoft apparently fixed something in .NET Framework 3.5 Service Pack 1 that previously didn't work, or somehow worked differently. As a result, Milkify will crash if used on a machine that doesn't have this service pack installed.
What you get is a XamlParseException that says that a ContentStringFormat property cannot be converted into a TemplateBindingExtension. This is because Milkify is using its own WPF skin where, among others, the check box control has a new custom template.