You might find there is some software on the installation CD that doesn't run on 98se, but it is usually a bad idea to use an ISP's CD anyway as the software they install isn't needed to run your actual ADSL connection, it is just other rubbish that slows your machine down or is just a waste of time. The only useful thing the CD does is set up your modem/router to connect.
As long as you can establish the settings for your modem/router you will be able to install/setup those yourself. If you post your make/model of modem/router someone will probably be able to tell you the settings.