Comcast (in Sunnyvale, CA, anyway) requires no special software to get it working, nor does it require registering a MAC address. When the installer is onsite to install, he'll set IE up with a specific proxy address and then log into a specific attbi.com website where he'll manually activate the account by filling out an online form that ties the specific cable modem to the cable internet subscriber account. After that, it works by standard dhcp, and you can change the connection from the PC to a router without special configuration.
I never installed the software (my ex g/f's parents did, and they're complaining of longer bootup times). There's a LOT of junk on the Comcast software CD, lots of misc DLL files and whatnot, with no clear description of what it all is or what it all does.