PC specs are:

AMD Athlon 64 3200+
ATI Radeon X800XL 512mb (catalyst 7.9)
2GB ram
Dual monitors (not sure if this could be a problem)
Vista Ultimate with all windows updates

Ok so I did a complete install from the DVD before I tried to play but I can't get furthur than the start of the Cairo level before it crashes to desktop with no specific error, just the generic "Halo 2 for vista has stopped working etc.". I thought it might be a memory error so I disabled everything non-MS, even my AV's realtime protection and increased the pagefile to 3GB but it still crashes. Im busy defragging now as per the other posts advice.

Any help would be greatly appreciated