This means the version of that DLL file you have is not compatible with the version that Halo 2 wants to use. You have no choice but to replace it if you want to play. OS's don't downgrade themselves so I guess it's a newer version of that file that makes it incompatible since it just started happening. If you put that file in your halo 2 folder, it should load it first before even looking in the system 32 folder.
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