Leave your phone unplugged from your computer
Start PC Companion
It will say phone not found and give you a repair link
Click on that
Accept that you will lose all data
Select the model of your phone from the popup
It will hopefully ask you to connect your phone in flashmode
Do the following:
Attach your phone to the computer using your usb-cable
Press HOME-POWER-BACK-buttons all at once. (your screen will turn off because it goes into reboot)
Release HOME and POWER but keep the BACK-button pressed.
It should go into flash-mode and PC Companion should start flashing the firmware.