Go here for direct avast! support and start your own new topic: http://forum.avast.com/index.php?PHPSESSID=ltf0gj6tfradqof1ae1l9ne884&board=2.0
You're not the only one with this issue, plus you will need to register to post over there. If you decide to do this, please mark this thread as closed, and continue over at avast! forum.
If you do decide to use system restore to go back to where avast! was working, be sure to disable the self-protection module within avast! first before running restore.
avast gui>Avast! settings>Troubleshooting>Clear the box for "Enable avast! self-defense module".
Then reboot to set that setting first before doing the system restore. Self-defense module will see that avast! is in a new environment when you restore, and shut avast! down to protect it unless you turn it off first.
I was using avast 7 but soon they prompted me to update it to 8. I did but after installing and restarting...they prompted me to pay for full security. I wanted to use the free home version and therefore ignored it. But it soon stopped working all of a sudden. I downloaded a fresh installer and tried but it stopped installing in the midway, advising me to clear the cache and download it once again. I tried but nothing happened. I thought if i could delete the old files in the C drive and try but they are denying access to delete it although I am an administrator(in fact the one and only) of my PC. I run windows 7....2gb RAM...and a core 2 duo processor.. can anyone help..?