I'll throw the towel... man, what's going on in your computer?
Is there any other security application running? Did you block access to anything *download.avast* into your browser settings?
Are you using PeerGuardian or any other IP blocker?
I suppose you've downloaded the avast setup from its website...
Can you check the folder access permitions? (C:\Program Files\Alwil Software\Avast4\)
Hope someone from Alwil team discover this mistery