Check the Network Control Panel. TCP/IP should be installed. If it's not, then this would explain the problem. If there are multiple copies of TCP/IP bound to the same adapter (Dial-up Adapter) this could explain the problem. Remove one of the entries.

If TCP/IP is installed check its properties. Ideally nothing is set. If items have been set, be sure that no gateway address is entered. Be sure that if DNS information is entered that it is correct (i.e 128.223.32.35, 128.223.60.22, 128.223.6.9 for the UO). Use "winipcfg" by going to the Start menu and choosing "Run..." to verify that your machine has an IP address for the PPP Adapter.

If a proxy server has been set (see the Internet Control Panel) check that the domain settings in the TCP/IP properties is either blank or set to "uoregon.edu"

If the user has only tried one address have them try another. Maybe the server is down.

Use something like ping or traceroute to verify network activity.

Check the http://ns.uoregon.edu/status web pages to verify that the UO network is up.

Etc. You are likely to come up with more.