This may be off-topic but related anyway.
I now have exactly the described problems with my WinXP pc's. On my Win7 pc's, I don't use Avast, so no clue about that.
The difference from the other posts here, is that my XP's are standalone pc's with Avast free v7.0.1474.
The definition updating stopped working on the 11. october 2015, and afterwards the symptoms are exactly as described earlier in this thread.
The reason for writing this, is that I think the problem is in the definition signatures, since my program has not been updated.
I checked the ashupd.exe, and it is dated 30oct2012. The other files are also old, so the problem is not related to updated program files.
Of course I don't expect Avast to care about definition update bugs introduced to version 7.0.1474, so this post is only made in the hope that it may help
the IT-admins, who now suffer from this major bug, to find a solution.
And to the Evangelists: Yes, I know that WinXP and Avast 7 are are extremely dangerous, and should be updated