Hi mypyrex, wanted to let you know that you are not alone! I don't think this is an upgrade problem but a general problem with the 2014.9 update for anyone not running Windows 8, but I could be wrong.
I've had the same problems on my laptop and my mother's laptop - programs not opening, very slow operation, even SafeBrowser wasn't functioning properly, and the Task Manager wouldn't open, just kept putting little green boxes in the bottom bar. Oh, and I couldn't watch online videos after the initial program update although watching video worked after doing a clean install.
All was fine on both computers before updating to 2014.9. I run Vista and my mum has Windows 7. We have Avast Internet Security and have used it for years with no issues. No other firewall was enabled. After the initial updating from Avast (not clean) and having the above mentioned problems, I've done two clean installs now on my computer and one on my mother's. A clean install seemed to work better for a few hours but then found certain programs wouldn't open - Malwarebytes (free), TDSSKiller, and a photography progam - and things were hanging up again. These programs all worked fine before the update. SafeBrowser had some problem with shockwave which i don't know how to fix. It never had that problem before either.
So out of major frustration, I've completely uninstalled Avast for the third time. My laptop now works fine again!! I will leave Avast off both computers and wait until Avast (hopefully) fixes the glitches. Annoying since I have many months left of my subscription and I used the SafeBrowser a lot