Failed object initialization (ISupportInitialize.EndInit). Error on Windows XP

Failed object initialization (ISupportInitialize.EndInit). Method not found: 'System.Windows.Threading.DispatcherOperation System.Windows.Threading.Dispatcher.BeginInvoke(System.Delegate, System.Object[])'

If you get this error, looking at the MSDN page for the method call

http://msdn.microsoft.com/en-us/library/cc190824(VS.90).aspx

It says at the bottom:

Version Information
.NET Framework
Supported in: 3.5 SP1, 3.0 SP2

The computer was running 3.5 vanilla, install the update and you'll be good to go.