First I just wanted to say that I love Avast and have been a user of it for almost 4 years now. beforehand I used to use the likes of McAfee & Norton which were horrible and when I was told of Avast I was pleasantly surprised to see such an amazing Anti-Virus that was free no less!
I even went through that whole debacle months ago where Avast thought everything was a virus and kept blocking them off, but despite that I stuck with it even until now.
So here’s my issue: Ever since I updated my old Avast 4.8 to Avast 5, my MSN Messenger no longer seems to work or stay on. For example I would turn on the program, it would log me in for like 2-3 seconds then immediately be shut off after. I am sure this issue comes from Avast as that day Avast was the only thing I downloaded and it happened right after I had to reboot to fully install my Avast.
beforehand I used to use the likes of McAfee & Norton
did you use a removal tool to remove all leftovers after McAfee and Norton?
you can find it here http://uninstallers.blogspot.com/
If this does not help i would do a new clean install,
using avast removal tool and first uninstalling avast 5 reboot then avast 4.8 reboot and install avast 5 again