Hi Balack,
Here is what you need to do to completely remove the drivers from your computer.
1. Uninstall normally the current drivers from add/remove programs.
2. Downlaod total uninstaller trial version will do the job, and install it.
3. Now let total uninstall take a snapshot of your drive and registry
4. Now install the drivers again for the usb modem.
5. Use total install it will notice all the changes it had made to your computer.
6. Finally uninstall the drivers using total uninstall.