I fix a friend's pc it works fine at my house but when he got home he could not connect to the internet, but it works at my house. What do you think could it be the modem its with at&t and he has had it for about 3 years.

Try a different ethernet cable and resetting the router, or if his internet provider supplied him with a disc to set up his connection use that, or could be just a bad router.

Popeye made some good suggestions.
You might go to Internet Options - Connections - LAN Settings - make sure "Automatically Detect Settings IS checked, and make sure the Proxy setting is UNchecked.
Go into device manager and make sure your LAN/ethernet adapter is working ok.
If all hardware seems to check out ok, call the internet provider and ask them to guide you through the internet settings that may have gotten changed.

Try turning off the modem, router and PC and then in this order, turn them back on....

Modem first (Wait 20-30 seconds while it boots up)
Router Second (Wait for it to boot up too)

This allows them to have a proper handshake between them...

Then turn on the computer. Make sure it is set up for dhcp and you don't have a static address in it...

Good Luck...

Was he able to connect to his AT&T modem before you fixed it? If your repair was not related to his connection problem then it sounds like a setting. Is he trying to connect hard-wired directly to the router or is this wireless router?

If hard wired, recycle the router, reboot the computer and also verify that you get a green light in the back of the computer where the lan cable is connected. Also, verify that your proxy settings in the options section of your internet broswer is checked as well.