1. Use an Ethernet cable to connect the desktop to the Comcast Cable Modem.
2 Power cycle the Cable modem or use the Reset button in the little hole in the back.
3 Power cycle the Desktop
4 Ctrl Alt F1 to go to the command line (Use Ctrl Alt F7 to get back)
5 At the $ prompt, type:
$ ifup eth0
$ ip route
192.168.1.0/24 dev eth0 proto kernel scope link src 192.168.1.103
default via 192.168.1.1 dev eth0
If you see output similar to above then src is your IP address and default via address is your default gateway.
If you have an IP address as above then ping the default gateway (Use ctrl C to break out)
If the default gateway responds, ping Comcast.net and see if you get a response
ping 192.168.1.1 --- or whatever your default gateway is
ping comcast.net ---- or yahoo.com