http://openra.res0l.net/
EA released Red Alert 1 and CnC for free in the 10th anniversary of CnC series. However the installer EA offered does not support 64bit windows.
A community project,OpenRA (link above) offers the multiplayer /skirmish component of Red Alert 1 and Command and Conquer for free. The game is playable on linux, mac and windows 64 bit. It comes with the sound effects and graphics file, however the original music soundtrack is not included and will require the use of the game's original CDs.