Though there could be many reasons for this, one thing to check is to make sure there is no corruption with multiple modems created in Windows.
It is advised to every so often to clean up the modem assignments in Windows.
1) Go to control panel in windows
2) Select "Phone and Modem"
3) Go to "Modems" tab
4) If there is an extensive list of Modems listed it is advised to delete older unused ones.