For those of you that are still looking to tweak or to try and get better performance from your machine for Halo PC. I will not be responsible if you tank your PC but this is straight forward. Open dxdiag under the RUN command in XP to access some of the info. Get a friend if you need one before you try it if you are unsure. BUT again it is easy to do.
http://hce.halomaps.org/index.cfm?nid=308Ð