Microsoft currently does "not have plans" to release any of the Halo games on Steam, according to an official statement from Microsoft provided to GameSpot.
Halo: Combat Evolved, Halo 2 and Halo 3 were recently discovered in a public version of the Steam Database, suggesting that all three titles might be headed for Steam release. A Steam Community Hub for a Halo 3 was also spotted, but later removed.
Of the other games listed, Dyad has since been confirmed for a Steam release.
Halo: Combat Evolved and Halo 2 are already available on PC. Halo 3, however, remains an Xbox 360 exclusive.