Sorry some of this may sound condescending but is not meant to.
Did you make sure to fully uninstall the previous driver for your Hauppauge 1600? AND run the hcwclear program/script? and then restart before installing the new downloaded hauppauge 4.6A+ driver?
Did you restart after running the reg key?
Did you let the computer fully complete the search for digital signals? Mine didn't find any until the last like 25% of it's search.
Sorry that's about the best I can do without just saying, try reinstalling the drivers. I'm not sure about reinstalling the regkey, I don't know what that would do BUT if W7MC is seeing the QAM in the first place you probably have the reg key installed correctly.
Actually, do the QAM channels show up when you have your TV (direct cable connection, bypassing PC) search for digital channels?