Command & Conquer is one of the very first multiplayer real time strategy games that made a lasting impression with the gaming community. The very first DOS version of Command & Conquer that required you to dial another player directly or play over IPX with 4 players.
Command & Conquer: Red Alert changed the game completely with the introduction of Westwood Chat, which allowed you to play with other players from the internet or with 8 friends over IPX.
EA Games has released a free to download full version of both of these games as well as Command & Conquer Tiberian Sun. You can download the .ISO images from: http://www.commandandconquer.com/classic
There are some installation instructions located over at CNCWorld for Windows XP and Vista. I’ve had some trouble getting the original command & conquer to work on my Windows 7 machine, but I will try to find some instructions for that OS and post them below at a later date.
Install Instructions for C&C 95 (Original Command & Conquer) on Windows XP: http://www.cncworld.org/?page=features/tutorials/tdfree
Install Instructions for Command & Conquer Red Alert on Windows XP: http://www.cncworld.org/?page=features/tutorials/rafree
Install and Online Play Instructions for Command & Conquer Tiberian Sun Windows XP: http://na.llnet.cnc3tv.ea.com/u/f/eagames/cnc3/cnc3tv/TibSunFreeware_install.html
Most of these games will require the IPX protocol to play with your friends on a local network, as these componets are not installed by default on Windows XP, Vista or 7.