Comodo Dragon Internet Security V

I am running Windows 10 64 bit,
I have Comodo Dragon Internet Security installed and running smoothly for years.
I tried to install Bitdefender Anti Virus 2017 but after the installation and rebooting the computer when I logon to Windows I get a black screen with the Comodo gadget that is always on my home screen and that’s it. I can open Comodo from the gadget and see that Bitdefender is not recognized but beyond that nothing doing. I realize I should have tuned off Comodo but it’s too late now. What can I do short of redoing my C: drive?

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