USR Modem installs only on XP Admin acct

When I installed my US Robotics V92 Winmodem on WIN XP Pro it is recognized by the Administrators account only. When I log on as another user Windows says it can't find a modem. If I change the user account to an administrator account it finds the modem and works fine but then it's missing in my admin account. My ISP Net zero swears it is a hardware issue but yet says that Net zero has to be installed on an Administrator account and that other users have to sign up for their own account if they want to log onto the net. Any suggestions?