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Try this:
If you installed ADB VCOM drivers, uninstall them in Windows CP. Then uninstall USB device IDs of your phone using USBdeview
[Only registered and activated users can see links Click here to register]. run SP flash tool let Windows search and install the necessary drivers.
Although, I agree with cookiedough here. Usually the trick with the battery out is most effective. For the hard buttons, vol down is not the only possible one. you can try the others or even a combination of buttons.