According to Halo creators Bungie the PC version of Halo 3 (which hasnt even been announced but will clearly happen) hasnt even had one line of code written for it yet. It will clearly be created for Windows Vista at some point and also feature cross-platform online play with Xbox 360 over Live. Bungies Frank O'Connor had this to say about it:
Read Whole Article Here: Halo 3 on PC not in development... yet - CVG.com
Frank O'Connor said:I can tell you for a fact that there's not one line of code written for a PC version at this point. We're not working on it, Microsoft hasn't even chatted to us about it yet. They realise the important thing is to get the 360 version [out] at this point.
Read Whole Article Here: Halo 3 on PC not in development... yet - CVG.com