I got an Ext HDD for backup. During the installation of Avast5, I found it created some temp folder in my Ext HDD (not C Drive??). And then the installation freeze at “vcredist_x86_SP1.exe”.
After I removed the ext HDD, the installation run very smooth. No problem any more.
I’m using Vista Business 64bit SP1 (New installation). The new Avast gave me a bad impression because of this simple error.
I forgot to say… I can’t use mail protection before (upgraded from 4 to 5), with the same computer, OS & ext HDD.
My system run slowly after this, some software can’t startup, Windows update error… I thought I got a virus or malware at that time. So I reinstall my Vista.
What’s your question…?? 
asyn
Exteranal HDDs are for storage purpose only, NOT for program installations.
Are you sure the temp folder made by Avast installation? Not by other cause? Most applications using Windows temp folder or User Data temp folder or even it’s own temp folder when unpacking it’s installation and data file no matter if you store the installer file (exe/msi) in your external HDD and clicking the installer file from there.