usually in a case where my customers (or installing technicians) are unable to connect to a website I run through a pretty small list of things....
first things first... do they have a valid IP address? if you're doing support for a specific ISP, then I'm sure you know the range of IP's used.
to check these, use WINIPCFG in win9x, or IPCONFIG from the command prompt in winNT, 2k, and XP
second thing is have them try http://www.google.com
in IE. if they're unable to reach these, we check connection settings by going to the Tools Menu and selecting internet options. make sure all proxies are disabled for the time being.
if still unable to access websites, I have them pull up a command prompt. from here, we try to ping http://www.google.com.
this is usually where you get the tell tale info. if the system comes up with "Unknown Host" or something along that line, I have them ping the IP address of said site. in google's case, I have them ping 18.104.22.168.
if person on the phone is able to ping by IP address and not by FQDN, something's hosed in the DNS resolver in the TCP/IP stack.
if they're unable to ping by IP address, it's generally a firewall, bad IP, or bad stack.
if all firewalls are disabled, they have a valid IP address, IE's settings are correct, etc, etc, then it's time to start playing with the stack.
There are some light registry hacks that you can perform to fix this in win9x and win2k... for WinXP, because TCP/IP is a core component of the OS (and can't be uninstalled), the official command to repair the TCP/IP stack is
netsh int ip reset c:\reset.log
I've not had any luck with repairing the TCP/IP stack in a windows XP system. there is also an exe file available over the internet called winsockfix.exe do a google search for it.
feel free to e-mail me for directions on the win9x thing.. I have a small guide with pictures typed up for it at the office.. I'd be happy to send it.