Some of the older USR modems had jumpers on them to set the resources used - i.e. COM2/IRQ3. With these type of modem, you normally have to run the Add New Hardware applet and let Windows search for the port after you've deleted the port. After Windows finds the "new" port, reboot, then install the modem drivers.
But, if the modem was working fine, then, with no changes to the system, it started acting up, it may be time for a new modem.
If you're using a PS/2 mouse instead of a serial mouse, you could try disabling the onboard COM ports (in the CMOS
setup) and reinstalling the modem. After disabling the onboard COM ports, start Windows in safe mode and delete/uninstall all references to the modem and COM ports. Reboot and run the Add New Hardware app, reboot and install the modem drivers.