Done all that, also shop-lifted (edited) the registry for pdf955 (or PD955 as the BIAB pop-up, see OP above) entries. Even used the paid CCleaner (pro version). I guess it has to do with some missing windows (permissions?) setting. BTW i used Tweaking.com's Windows Repair on all sorts of possible installer issues. Afraid though to turn it loose on permissions.

BUT EUREKA: Thanks Kent, you made me re-think: got it working. Switched to my Administrator account. Installed from there, running as administrator probably wasn't enough LoL. F