Might be worth clearing out all old instances of the modem drivers in device manager
If you set the DEVMGR_SHOW_NONPRESENT_DEVICES environment variable, Device Manager will show you the "ghosts" of past devices that are no longer connected to your system (or in the case of USB devices, were perhaps attached to a different USB jack) if you enable Show Hidden Devices. From there, you can right-click and uninstall all these non-present devices. (Don't worry, the OS will detect and reinstall the drivers automatically when devices are reconnected.)
1. Click Start, point to All Programs, point to Accessories, and then click Command Prompt.
2. At a command prompt, type the following command , and then press ENTER:
3. Type the following command a command prompt, and then press ENTER:
4. Troubleshoot the devices and drivers in Device Manager.
NOTE: Click Show hidden devices on the View menu in Device Managers before you can see devices that are not connected to the computer.
5. When you finish troubleshooting, close Device Manager.
6. Type exit at the command prompt.
Note that when you close the command prompt window, Window clears the devmgr_show_nonpresent_devices=1 variable that you set in step 2 and prevents ghosted devices from being displayed when you click Show hidden devices.
Then try a reinstall of the modem drivers.