This is often caused by one of your control templates using something from the PresentationFramework.Aero assembly. This can happen if you modify templates by using "Edit a Copy" in Blend. This will generate a XAML copy of the currently active template for the system Theme under which Blend is running. In the case of Vista or Win7 that's Aero by default (Classic for XP). Many of the default Aero templates contain custom elements (ButtonChrome, etc.) that are specific to the Aero theme and declared in the Aero assembly. When running on XP the Aero assembly usually isn't available and so you get missing references in your XAML at runtime.
Do a text search on your whole solution for "PresentationFramework.Aero" and you should find some xmlns declarations using it.