I've had problems loading various software in the past. Not necessary modems but it could be linked! Often when you install something there is a box to be ticked which is asking whether this installation is for adminsistator only OR all users. Now if you miss that box when you log on under a different user the program cannot be used.
Had this once where I had to make every user an administrator to install the program and then had to reduce their user rights after installation. Hve not got a clue why, but at the time it was a workaround that did the job!