When I try to launch SourceTree it fails to launch and this exception is logged in event viewer:

Application: SourceTree.exe
Framework Version: v4.0.30319
Description: The process was terminated due to an unhandled exception.
Exception Info: System.Windows.Markup.XamlParseException
Stack:
at System.Windows.Markup.WpfXamlLoader.Load(System.Xaml.XamlReader, System.Xaml.IXamlObjectWriterFactory, Boolean, System.Object, System.Xaml.XamlObjectWriterSettings, System.Uri)
at System.Windows.Markup.WpfXamlLoader.LoadBaml(System.Xaml.XamlReader, Boolean, System.Object, System.Xaml.Permissions.XamlAccessLevel, System.Uri)
at System.Windows.Markup.XamlReader.LoadBaml(System.IO.Stream, System.Windows.Markup.ParserContext, System.Object, Boolean)
at System.Windows.Application.LoadComponent(System.Object, System.Uri)
at SourceTree.App.Main()