This is a tough problem unless you are really familiar with networking or have someone to help check some things on your computer & router. Some things you should check are your Windows firewall settings, DHCP settings. A tutorial on how to do this can be found here. Did you setup this computer for your home network and/or do you have the authority on the computer to do so?
I agree with Labnuke99 that this can be tricky if you’re not familiar with network setup. Comcast *might* be able to help if you contact them. The fact that connecting directly to the modem with the PC suggests that your PC is nor set up for DHCP.
If you’re feeling brave you can try this:
Right click on network neighborhood,
click on properties in the drop down.
select the appropriate network connection and right click on it
select properties from the drop down
select the Internet protocol TCP/IP setting from the list (you might have to scroll down)
*** Write down any settings you find here!!! Just in case you have to put them back!!! **************
Select Obtain IP address automatically AND Obtain DNS server address automatically.
Thank SHOULD take care of it.