Comcast modem why the pc/activity light always flash?always make me disline

First make sure your router is working (can you ping it?)
Open the modem's web page at 192.168.100.1 and check the Status. Also look at the Signal-to-Noise ratio. Check the logs for obvious errors.
Do NOT change any settings if you don't know what your doing!
If you have exchanged the modem, comcast has to know the new MAC address or your modem will not authenticate.

Either post your findings here or call comcast again - there might be an issue with the connection to the outside, since you are (hopefully) getting an IP address on your computer and the modem is powering on.
Question always is: is this your modem or did you lease it from Comcast? If it's theirs, they have to fix it, if it's yours you can call and tell them that internally everything is working.

Let me know if you need more help.

call comcast to activate your modem.
US amber indicates 100Mbps connection.

either:
1. call comcast to have them activate your modem
or
2. power cycle your modem to make it recognize your router or computer

failed on loading skin name "comcast". the file "shell.css is required in your "sscommon/skins/comcast/css/" directory" this is what i am getting from my acer desk top.... comcast is my internet service provider and say "everything is fine on their end" but i cant access the net... please help me

My RCA DCM425C from Comcast had the same problem when I came home Saturday, 11/14/09 (coincidentally the same day you posted). Had been working fine - cable TV still works fine including On Demand, etc. The Internet light was blinking. Occasionally I'd see it try to work from one light to three or four, but it always came back to one. Called Comcast, they said their side showed "no signal". Sent out a tech today. He replaced the cable modem, reprovisioned it (I guess if I had a backup ready I could have done the provisioning over the phone maybe?)), everything's great. Lasted about a year and a half. When I logged in via http://192.168.100.1, it said it was still in Step 1 of 5 ("Tuning"), and the logs showed a whole bunch of errors on Saturday that stopped after that. Diagnostics would always freeze the built-in web server and require reboot.

If you have the following lights :
Power light - on
Receive light - flashing
Send light - flashing
Online light - off
PC/Activity - on
Standby - off
Then your Network Access is OFF - modem is successfuly registered but Network Access Control is disabled via DOCSIS configuration file.

Call your internet service provider....

Comcast is full of crappy there internet connection is horrible they are lying to everybody take it back like I'm doing I have never had such horrible connection my cell phone from ATT internet has never not once connected but for some reason my Comcast never connects

are you sure your hooking the comcast modem in the right place?

Are you running the setup disc provided by comcast to get your email setup?

If you are , you do not really need that disc as it provides useless and often outdated junk on the disc. You should be able to surf the internet by plugging in your Ethernet Cord into the laptop with no problem.

On the issue of the Comcast user name and password, this was originally used at the original configuration of the modem when it is wallgardened.

You can always call Comcast and have then activate the modem for you. You'll need the Mac address at the bottom of your modem to activate it over the phone.

Online Activation usually requires your customer account number. But since it looks like you do not have this data, you will need to call them directly.

Only thing to do is buy a new modem. Mine did the same after a power surge. Left it all night off and in the morning started working again.
a month later, another power surge and never started egain. Just got me a new one and called Comcast to activate it. Problem solved.
Good luck there