I recently upgraded my Avast free to version 5. When I tried a full system it crashed and the shields had to be reset. I tried several installs and the problem persisted. I finally found the problem was Webroot Spy Sweeper. I also had the same problem when I tested it on a virtual PC.
I came accross very similar problems with Avast 5. Unfortunately Avast 5 does not seem to be as good as Avast 4.8.
I personaly downloaded 5 and found it to be somewhat unstable. After looking on the Avast forums it seems that I am not alone.
After reading several pages of problems with this new flavour of Avast I decided to revert back to 4.8 and things have been fine ever since.
I have used Avast for several years now with no issues but if I have to go to 5 and the bugs are not ironed out then I will go elsewhere.