If it's Running XP ->
1. Head into the Control Panel
2. Go to Network and Internet Connections
3. Head to Network Connections
4. From here, run the "Network Setup Wizard".
5. If this the only computer on the network, click "Next" 2x
6. Select the "Other" radio button.
7. Select the Middle Option.
8. Hit Next.
9. Let it do it's thing.
10. When done, hit finished. (If using SP2, you will be asked to Turn Off File/Print sharing. If only computer on the 'net, then turn it OFF.)
Try it now. What this will do is clear out the network setting, and restore it to it's "Auto Obtain IP", and it's defualt.
Don'f forget Comcast's "FAQs" ->
http://faq.comcast.net/faq/query.jsp?name=Connection