Halo 2 has cache content in E:\Cache and X, Y and Z drives.
Try clearing your X, Y and Z drives, and the aforementioned cache folder, and see if that fixes your problem.
I personally wouldn't lay it on a dying HDD, seeing as your other games work fine... A friend of mine had similar problems (maps took ages to load, loading bar at startup) and it was a simple cache delete which did the trick.
Hope this helps.
EHosin
p.s. If you want to delete the map packs/halo 2 saves, you can do that in the memory section of the Xbox Dashboard.
Did that and same results. Maybe I didn't get the map saves though (was looking for Halo 2 alone)
I'm thinking the auto installer ticked something off.
I don't think it's my hd either because I was trying out a lot of games and xbmc last night and everything is fast and working except Halo.
Thanks for your help and suggestions.
Now I'm gonna be all rusty in Halo2 and die a lot on kai.