"Letter head" is likely the tray you've loaded letterhead into.
Take a look at the PrintDocument's PrinterSettings property under PaperSources. Create an algorithm to choose the proper one, and set it's value to the value PaperSource under the document's DefaultPageSettings property.
Something like this, although you'll have to figure out
which paper source to use, and this is completely dependent on your printer and it's setup.
document.DefaultPageSettings.PaperSource = document.PrinterSettings.PaperSources[0];
David Morton -
http://blog.davemorton.net/