I managed to get C&C DOS multiplayer working across the internet on XP & Vista with another friend. It's very easy and all you need is the excellent fast ykhwong DOSBox 0.66 build at http://doom.vachu.com/DOSBoxEasy.exe and no need to worry about firewall settings or complicated setup.
ykhwong's DOSBox has been specially modified/enhanced & runs complex DOS games super fast even on low spec P3s & P4s.
We're looking for other players so we can get a 4 player C&C DOS games going... and the long-lost C&C Capture The Flag games.
Reply here or add my MSN to your contacts list (which you can figure out from my username).