That's the problem, avast doesn't block, it scans and alerts to infection, but it doesn't block. If there is an apparent blocking, this is more likely to be conflict, normally associated with other security software or remnants left behind. So removing one side of any conflict would appear to resolve the problem, but it doesn't identify the true culprit.
Have (or did) you another Anti-Virus installed in this system, if so what was it and how did you get rid of it ?
What other security based software do you have installed, anti-spyware, firewall, etc. ?