See related articles here on SO:

Injecting Mouse Input in WPF Applications

Simulate Mouse/Keyboard Input In WPF

Move the mouse in wpf

EDIT: concerning the specific "drag" need, here is another link (from the NUnitForms project here: http://nunitforms.sourceforge.net/) about a MouseController utility code that contains mouse simulation methods:

http://nunitforms.svn.sourceforge.net/viewvc/nunitforms/trunk/nunitforms/source/NUnitForms/MouseController.cs?view=markup

It has a Drag method. You could test if this works. I know, it's especially designed for WPF, but it's worth a try. Plus, if it's only about mouse mouvement, I don't see a problem if you need to reference Winforms assembly.