Guys, since there is a number of pending threads about this topic and it's quite strenuous to keep them all updated, I've decided to start yet another thread with the anticipation that we're close to the final solution.
My impression is that the problems are caused by a bug in the updater that sometimes fails to install the new driver files. That is, the updated drivers are correctly downloaded and unpacked, but are not installed to the system. Since other avast files are installed properly, there is a version-discrepancy problem (the program and driver versions do not match) that may eventually lead to disasters.
I'd need you to check if the drivers (i.e. the *.sys files) located in the <avast>\setup\inf directory are the same as these in \windows\system32\drivers (with respect to file sizes, timestamps etc.). They should be the same. If they're not, there is a problem and you should copy the <avast>\setup\inf versions to \windows\system32\drivers.
Please keep me informed - I would really like to know whether this is the real cause of these problems.
Thanks,
Vlk