In anticipation for Command & Conquer 4, EA have released free versions of a few of the early Command & Conquer series. It is completely free and patched to work on more version of Windows besides Windows 95.
The free games include Command & Conquer: Red Alert, Command & Conquer: Tiberium Dawn, Command & Conquer: Tiberium Sun and it's expansion Firestorm. Generals is not in that list, I'm afraid as they released classics related to the Tiberium series.
