As part of C&C's 12th anniversary, you can download C&C Gold in full (both the NOD and GDI discs) for free from the game's website:
http://www.commandandconquer.com/
Alternatively, if you have problems with the site as in my case, you can get the 2 iso files here:
http://www.shacknews.com/onearticle.x/48749
The game is a true classic and defined the RTS genre and I'm sure like me, others have fond memories of playing the original version of the game on MS-DOS (still have the CDs lying around somewhere ). The Gold edition was released later to provide Win 95 compatibility, SVGA graphics and internet play.
This version of C&C Gold is XP compatible but I'm not sure if it works on Vista.
Well worth a download, despite it's age and afterall it's free