Pondus
2
are you sure all remanents of previous used AV are gone?
have you run the vendors removal tools?
General: Uninstalling a third-party antivirus software
http://www.avast.com/faq.php?article=AVKB11
have you tried avast repair?
controlpanel > ad/remove programs > avast > unnnstall > repair option…wait a minute and reboot