Apparently with Windows Vista DDE is no longer used as seen by this link:

NetDDE—For security reasons, Windows Vista does not support NetDDE. (NetDDE is disabled by default on Windows XP SP 2 and Windows Server 2003.) Regular DDE is still supported. NetDDE is a technology that allows applications that use the DDE transport to transparently exchange data over a network. The result is the application fails to exchange data over the network. To workaround, use a different networking technology, such as DCOM or Windows Communication Foundation. For more information about NetDDE, see;en-us;125703.