Hello, I'm an average end-user. My computer was top-of-the-line back in 03. I was playing the Halo campaign the other day and noticed the strain that was put upon my computer.

Most computers at the time wouldn't have been able to take the strain of rendering twice the load at once. At some points the XBox can't take it; you can see some slowdown in some spots where there is clearly being too much rendered at once.

If you want co-op so badly, go play the XBox. Consoles were meant for multi-player use. The PC was created for just one person.