Hello,
The last version (downloaded a few minutes ago) refuses to install: message is "the installer is unable ton initialize early avast self-defense with error 0x00000005! Aborting! ".
Can you help ?
Avast sometimes has issues for me when loading for the first time on a computer that is already infected.
If you have access to another computer that is already running Avast, perhaps you could try creating a rescue disk? (Click Tools, Rescue Disk). Running the bootable Rescue Disk may allow you to clean up enough bad stuff to allow Avast to install.
what AV did you use before installing avast?
have uninstalled it according to vendors instruction and using removal tool?
General: Uninstalling a third-party antivirus software
http://www.avast.com/en-eu/faq.php?article=AVKB11#artTitle