TV signal is a different signal, or frequency. In order to have Internet you're are going to need the external modem. The word Modem comes from the words Modulate and Demodulate. That process is necessary to receive the Internet.

You need the modem, it essentially takes the signal coming in on the cable and turns it into something your computer can use. You also need to pay Comcast for service, just plugging in any old modem won't work.

At the beginning of my relationship with Comcast cable, I purchased my own modem -- rather than continuing to pay $5 per month to use theirs. Visit your local electronics retailer and discuss modems.

cable modem!! not just any modem is correct! there are other types of modems. There are pros and cons to getting your own cable modem so be sure you do your research about that.

If what you want to do is watch tv on your pc there is a way to do that if that is what you are trying to accomplish let us know.

Cable Modem Primer
has some good info
Which is better, buying or renting a cable modem?
some companies do put special firmware in their modems which would not be in one you buy elsewhere.

Ok thanks y'all I Guess I will just keep the $5.00 a month rental box from comcast. I just thought there was an internal modem in my computer and I did not know how to hook it up. Thanks for your help.

no not for cable, some computers did come with a built in dial up modem, most of the newer ones don't anymore because not as many use dial up, however that kind of modem will not help you with using cable it has to have the cable modem.