Eyz ... Although you've given a thorough description of the problem along with all the background, there doesn't seem to be enough info to figure out exactly what is wrong. Sorry the Comcast tech made it worse.
By what you've said, I got the impression that the tech admitted defeat and was telling you to save your data and then re-install Windows. I hope that's not really necessary.
I'd start by tring to find your Comcast set-up/installation wizard and re-install their package just as if you just began your service with them. I feel this will straighten out any Comcast-related settings that got messed up, but it might also verify and/or correct other Windows-related settings that are needed to successfully access the Internet. If the Comcast installation wizard is really good, it might find and report whatever problems there may be.
However, if this doesn't fix it and you don't get the help you need from here, it seems you'll have only two options left:
1 - Bring the system to the Geek Squad or some other competent technician, or
2 - Save all your data as best you can and then re-install Windows.
Good luck, Eyz.