Boot in safe mode, launch IE and remove ALL browser helpers. I wonder if it's not installed as a BHO - so you'll need to get into safe mode, run Trojan Remover as well as the malwarebytes app you have run.
It's most likely indeed disabled SEP. You'll need to get into safe mode and remove any browser helpers IF that's what has happened. Don't even trust those that appear to be from Adobe - I've seen them claim to be Adobe, but in reality, not.
It's also possible you have a rootkit situation, if that's the case, you'll have little choice but to get into safe mode, administrator, run TR and manually delete the HIDDEN registry entries and hidden files and folders it's placed. TR will at least alert you to their existance.
I think Trojan Remover is from Simply Super Software. Sorry, Symantec, there are cases where we need to pull out the sixshooter as a shotgun may have a nice spread pattern, but limited distance. The other guns are more precisely aimed at specific targets.
A good mechanic always has another set of tools in the garage just in case.
IF you can successfully find and quarantine any files, do submit them to Symantec!