Finding a virus probably means that you don't have hardware problems.
Definitely run TDSSKiller.
See how your performance is doing afterward and if everything looks back to normal, (you're going to hate me for saying this) run Malwarebytes again, full scan on just C drive.
I ran the TDSSKiller, it scarcely took long at all and didn’t find anything. I also went ahead and ran the Malwarebytes again overnight (took just under 7 hours this time) and it spotted nothing.
However, I’m still having performance issues (slowdowns, etc.), although the frustrating concern of getting bumped off password-required sites I haven’t attempted to check, although I’m about to (oddly enough, it hasn’t happened on FR, but has on eBay, IMDb and other lesser-visited sites).