Stone, it should not being occuring this 
Did you try to install avast in another computer?
Have you close all the programs running before installing avast?
Have any other antivirus in the system?
Help us giving more information… With the one posted here, I cannot figure out what is happening…
Lee, I cannot imagine why you post this so sure :
Sometimes the order will bring trouble but not all the times and with all combinations on antivirus and firewalls… 8)