Take a look at the instructions given by DC3 that I provided the link to for clean boot.
That or you could try asking that this thread be moved to the "Am I infected? What should I do?" forum - as I'm convinced that something's very wrong but not knowing what may or may not have been installed or what else has been going on it could be an infection.
If the boot times on the computer seem normal, but the response once booted up is what's gone down the tubes I'd try the Clean Boot process to see if you can narrow it down to a given service as the cause. If you never get the machine to stop misbehaving after the first two "disable half the services" passes that means that something else is wrong. In which case you could try SFC and DISM to see if they find anything, but that's putting the cart before the horse right now.
Brian AKA Bri the Tech Guy (my website address is in my profile) Windows 10 Home, 64-bit, Version 1709, Build 16299
Here is a test to find out whether your mission in life is complete. If you’re alive, it isn’t.
~ Lauren Bacall