"so the likely explanation was that the toolbar had come onto my system piggybacking on the installation of one of the several video-card stress testing and temperature profiling tools I used while overclocking the system."
So why would a browser toolbar component become part of a video-card/temperature tool bundle? Poor DLL tracking on the tool developer's part?
Most often than not, it's adware explicitly included in the installer by the software's creator to make a profit on shareware or freeware.
They usually have opt-outs, but the shadier ones do not - and you can legitimately click "next-next-next" and don't read about them and uncheck the button that installs it - that's what they're counting on.
If someone like Russinovich didn't see it, it was probably not opt-out.
So why would a browser toolbar component become part of a video-card/temperature tool bundle? Poor DLL tracking on the tool developer's part?