Did your new machine come with a pre-installed trial antivirus (AV) on it?
Was your machine trying to do a Windows Update (WU) at the same time it was trying to install Avast?
What other security software do you have on the machine (firewall, other security software including trial software, and how did you uninstall them)?
Are all MS Updates current?
You really should update to Avast 6.0.1125, but I need to know the answer to the first question first.