Halo 2 Asking for disk everytime I finish the Quarantine Zone

Everytime I finish the Quarantine Zone level, and the cut scene plays where
the brutes shoot you off the edge, it comes up with a message "Please insert
your original Halo 2 disk". What I have done so far is uninstall/reinstall
of Halo 2, no luck. Decided I could use a fresh install of Windows anyway,
so reinstalled that, and then Halo 2, still no luck. Figured maybe there was
a problem with my disk, returned it to the store and exchanged for a new one.
Again, did a fresh install, played through to this point, and get the
message at the same point. I don't even get an option to do anything but say
ok, and it takes me back to the main menu. I've tried playing with and
without the disk in the drive. Inserting the disk only when it asks, and
ejecting and closing the drive, and it will not do anything with the disk,
even though it asks. I have tried everything I can think of, 2 installs of
Vista, and multiple fresh installs of Halo 2, and still no luck!