System hangup after phone disconnect

Whenever I log off the internet or end a fax session, after the call is disconnected my system freezes. This occurs whether I use communication software, fax software or dial-up-networking. I have a USRobotics 56k V.90 voice modem that I’ve been using for some time. I’ve re-installed the operating system and the communication software (Earthlink)with no success. I’ve also removed and re-installed the communications components of Windows ’98 with no success. The problem appears common to all COMsoftware. In fact when I finish this session I’ll have to reboot my system because it will lock up after I log off. Any suggestions will be welcomed. Thanks.