User can connect but no data transfer

We have several customers who are able to connect to the internet but move no information. It is not a pop problem, because we are able to move information through the pop in question. It is not a username/password issue because those are able tovalidate just fine. Specifying DNS numbers has not worked, nor has removing temporary internet files and cookies, or reinstalling TCP/IP protocols and microsoft clients. This error occurs on a variety of windows platforms. Replacing of winsock fileson 98 has also not worked. Any ideas?