My bad for not being completely clear, but when I made my last post, the symptoms had already returned, I wasn't simply anticipating that. I'm still seeing the ads on some pages.
I'm not sure if this is part of the problem, but just in case: when I ran the FRST fix, I accidentally forgot to make sure it was plugged in (I'm using a laptop) and it ended up running out of power. However, considering that the fixlog.txt was there when I booted it back up, it must have finished the fix before running out of power, so I assume(d) that that wasn't a problem.