I was able to reproduce this bug on a fresh installation of windows 7 ultimate 64-bit (no programs installed except avast) running on virtualbox (default settings)
all I did was first install Avast! Free Antivirus 5.0.677 (custom installation with system field shield, network shield and webshield). Reboot. Then update to 5.1.864 through the gui.
Reboot. System shield and network shield are not doing anything, except webshield.
looks like this affects everyone that is running windows 7 64-bit
EDIT : I tried to do a non-custom installation of avast 5.0.677 , and then update to 5.1.864 ...everything works normally-
looks like a custom-installation of 5.0.677 (with some shields disabled) does not update to 5.1.864 properly in win7 64-bit!