Get a wireless router, an older one like 802.11b cost less than $50 now, and one or two compatible adapters depending on how many computers you want to connect wirelessly; but it's always a good idea to keep one with wired connection for security reason.

Thanks. Does the main computer that Comcast comes through have to be on to access the internet on the wireless connected computer?

As long as I have the main computer connected to Comcast and set up with decent security, am I protected on the other computer? Do I need the same level of security on the other computer? 

You connect the cable modem to a port on the router.  You connect both computers to the router using network cables or if the router is wireless you can connect wirelessly instead.  There will be no "main" computer.  Each computer will be able to access the internet independently.  The router should have a built in firewall and should provide some protection.  Just to be super safe, I also run Zone Alarm on each computer.

If you go wirelessly, I highly recommend using 128bit WEP or WPA and MAC filtering.

Cliff