Ever tried porting a DDE app to Windows XP/2003 that you know worked under Windows NT/9x? It just won't communicate. Having found the problem twice and then forgotten again, here's the note on what is (normally) wrong:
The NetDDE services (Control Panel-Administrative Tools-Services) need to be started. Set startup type to automatic (they are disabled by default), and to avoid rebooting the first time, start them as well.