I have halo one and ther is no way to play as a grunt its just single player as mastercheif and then multiplayer as a spartan. Maybe you can in halo 2 or so forth or maybe there is some kind of mod but no, you can't play as a grunt.
You have to get Halo CE for the PC, and run the game in Dev mode.
Depends if you're playing Halo 1(Combat Evolved) or Halo 2. 1: Left CTRL 2: Don't know because I've never played 2 on the PC, sorry
I'm master chief
Skull Haven I play Halo 1 though
Oh well that easy! Direction: 1: First go to Google 2: Then type how to download halo PC or 2 3: Next, fnd the best place for you! 4: Download! 5: Done! 6: I download Halo PC at Free game download!