Try checking this:
Go to Control Panel - Phone and Modem Options - Modems - Properties - Modems - Maximum Port Speed. Change the maximum port speed to the highest option (115200). Also just try the connectors to the phone line and into the back of the modem.
I lost speed when i had multiple devices pluged into one telephone socket. My speed picked up after directly wiring the modem extension into the socket (although getting the wires in the right place was a pain).
Hope this helps