Comcast has been blocking port 80 (HTTP) for a while now, and they've recently started blocking port 25 (SMTP) as well. IMNSHO, it's only a matter of time before they start blocking all syn packets and charging extra for ANY incoming connection, but for now you can do it with some workarounds.
In short, if your flavor of Comcast is like mine, you can't host a server unless you pay extra or are willing to employ some dynamic port mapping service.
However, they do provide web hosting for customers, so if your aim is to host a web site and you don't care which computer it uses, that would work.
If you're more interested in operating your own server, I suggest you sign on as a junior system administrator at any organization that will have you: you'll have better hours than if you do it at home.
William, who was once woken at 3 AM by a System Admin in California who wanted to help me fix my mailing loop problem *RIGHT NOW*.