Unable to Acces GSM modem via usb to Serial cable

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embedded

Windows

Hi All,
      I am using GSM modem for my collage project. In my project I try to access that GSM modem by using USB-Serial cable. My Os is Windows 7(32 Bit) that ask the prolific usb to serial adapter Driver need to install but I intall so many drivers, but I am not able to access my GSM modem via X-CTU (Application that I used to access my modem by AT-commands) . My modem will return the text which I gave through AT-commands.
Eg;>AT (Command by me)
    >OK (Modem’s reply)

X-CTU app show that “prolific usb to serial adapter(Com9)” but I used to click ‘Test/Query’ button it will pop-up the window that shows “Unable to Connect ”
Please help me…

Regards,
lala boy