Nothing. Halo 2 fared badly on PC because Hired Gun ported it badly. It was a console game pretending to be a PC game, with numerous bugs and missing features. The modding also took a step back, from the Xbox version and the original Halo on PC. Halo PC showed that Halo can work on PC if ported correctly. Unfortunately, it seems that the Halo 2 port was rushed and released before it was truly a PC game for Windows Vista. I don't think after disappointing sales M$ will try again. I would like to see more Halo games on PC but it seems unlikely at this point.