Windows 8 user here as well, and I got the same error.
How about updating the Steam page to warn about this issue? I know it doesn't list Win 8 specifically, but this is the only game I've tried that does not work with Win 8. I've tried about 60 games, so I think It can be pretty safe to say this is an error specifically with COD and not Win 8. So a warning on the store page would be nice. Right now, with no official word from the devs, I feel like I've pretty much flushed 20€ down the drain.
Cheers
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