I have a trojan - I think --

Hi, I'm new here. I have some jerk who follows me around and does things to my computer. He likes to change my passwords and other annoying things like that. He's been trying to log into my bank account, but he can't do that since I switched from Malwarebytes to Bitdefender. Today I got a notification from Bitdefender that it was protecting the new device being installed on my computer, but I didn't install any new devices. How do I find this device, which might be something bad from the jerk who follows me around, and deal with it?

Thanks for any and all help.