Our 32 bit Vista machines started doing this recently. It is the "other" patch... KB3011780 - if it installs before avast gets updated to the latest engine, bad things start to happen.
Fix seems to be: Reboot the machine in safe mode with networking, uninstall KB3011780, do not reboot, go into the avast installer (from programs and features) and do an update there, while in safe mode with networking... let avast reboot the machine after it is done updating... reinstall KB3011780...