Yes, the Open Candy file was in just the place described ^.
I use CCleaner religiously at the end of each session but on this occasion I'd done a manual Windows update but not restarted the computer. As I understand it if I'd used CCleaner I would have deleted those update files before they were installed and had to have downloaded them again. So on this occasion I specifically didn't use CCleaner and went straight to my post updates/maintenance AV/AM scans where the Open Candy file was quickly reported as a PUP.
Also as said, I had no problem with Driver Booster when I first installed it from disc but unless this was something to do with AVAST's unwelcome flirtation with Open Candy it can only have come from the automatic Driver Booster update. It certainly wasn't there the last time I did a quick scan with MBAM the previous day and all the other manual updaters were, like everything I download, demand scanned by MBAM and AVAST immediately after downloading/before use.
The time of installation, earlier that day, also coincided with when I'd booted Driver Booster intending to update it manually and finding it had been set up to do so automatically when the program was running. By choice I switch off all automatic updates and only allow AVAST that privilege. I'm not even comfortable with that.
Driver Booster's automatic updater and not using CCleaner, for the reason described, was almost certainly the reason Open Candy sneaked onto my computer. If I'd had the control and followed my usual regime it wouldn't have happened.