If you see "USB Device not recognized," install BlackBerry Desktop Software first. It includes the necessary BlackBerry Virtual Serial Port driver.
You may still be able to use the legacy BlackBerry Desktop Software to reload the OS if you have a local copy of the firmware installed on your computer. blackberry 9790 flash file
A “flash file” (firmware image) is a packaged set of system software components that, when written to the device’s non-volatile memory, provide the operating system, core services, device drivers, radio/baseband firmware, and sometimes carrier-specific customizations. For the BlackBerry 9790 this includes: If you see "USB Device not recognized," install
To "flash" or install a new firmware piece on your 9790, follow these general steps: A “flash file” (firmware image) is a packaged
: The core utility used to push the firmware to the device. It is usually found in C:\Program Files (x86)\Common Files\Research In Motion\AppLoader after installing a firmware package.