Hey no worries, this is a common problem and I'm glad I can help.
The network you're setting up needs two essential things to make it work: An external IP address and an internal IP address scheme. Allow me to explain:
The external IP address is the one that is assigned to your router by your ISP (in this case Comcast). This is the address that all computers that are trying to send you data through the internet send it to. It's up to your router to determine which computer on your network requested that data. When you connect to the router to set it up and it asks you what type of IP address you are using most ISP's use Dynamic. This way, when you switch on your modem and it starts communicating with Comcast they end up setting up all the IP address configuration automatically. Manuel means you would have to set it up yourself. A few ISP's still use this method but not many.
The interal IP scheme is the addresses that are assigned to all the computers inside of your network. Most likely they are 192.168.1.* or 192.168.0.*
I would suggest trying to configure the router on the computer that already has Comcast installed. As long as you don't change any of her settings the worste you can do is misconfigure something on the router.
One thing to worry about: The physical ports on your router and each computer are assigned a MAC address. Many ISP's will LOCK the MAC address of the WAN port on your modem to your internet account. This is more of a security measure then anything else...if they suddenly detect a change in MAC address this *could* mean someone has hijacked your connection. 99% of the time it's simply because the end user has gone out and bought a new route (such as you did).
My suggestion would be to contact Comcast and speak with them. They can walk you over the phone step by step and are usually better then reading the manual
One thing to note: They will probably ask you what the MAC address of your router is. If you turn it upside down there should be a sticker on the bottom that tells you.