Invalid FORMATETC structure Error
Usually, you get this error when an assembly reference in your Visual Studio is messed up.
In my Visual Studio 2008, I was getting this error when drag and drop a control from Toolbox to a XAML page.
"The operation could not be completed. Invalid FORMATETC structure."
I right click on Toolbox and selected "Reset Toolbox" menu item. It resets the assembly and control references and I no longer get this error.