The location of the Desktop varies between different versions of Windows, and in a corporate environment can be moved around by the network administrator. On latest Windows versions, you can use the value of the USERPROFILE environment variable to find where the desktop might be:
C-x C-f $USERPROFILE/Desktop
If this doesn't work, then you probably have to forgo the keyboard just this once, and either drag a file onto the Emacs frame from the desktop, or use the file dialog (displayed when you use the toolbar or menu by default). Once you have a file from the Desktop inside Emacs, C-x C-f will quickly reveal where your desktop is kept.