Avast will not update definitions

[TheScorpion]

I have V. 5.0.418
Asus laptop
Windows XP pro
Comodo
Firefox.

I initially updated from 4.8 about a week ago and all has been well.
Today I found my definitions would not update.
I have tried auto and manual over a period of several hours.
The update process freezes on "D/loading file: servers.def.vpx"
It then eventually changes to "Cannot connect to d/load 697.avast.com" or "Cannot connect to server".
I tried uninstalling Avast and installing a new setup file. Same problem only now my definitions are over two weeks out of date.
I have tried d/loading with Comodo disabled, still no luck.
I have tried d/loading with Internet Explorer, still no luck.
All other connections to the internet seem to be ok.
Is this a bug with V5? Others seem to be having this problem too.
Any suggestions?

[olddog]

Try specifically deleting any permissions for Avast in the Firewall, then reboot.
When the firewall asks for permission for Avast items, make the permissions permanent.

There present permissions may be corrupt - the above will properly reinstate them. Stopping a firewall does not necessarily totally remove its effects from the system - that often requires a complete uninstall followed by a reboot.

The Firewall permission important to updating avast 5 is Avast! Antivirus Update ("C:\Program Files\Alwil Software\Avast5\Setup\avast.setup.)

The other two are Avast! Antivirus ("C:\Program Files\Alwil Software\Avast5\AvastUI.exe.) and Avast! Service ("C:\Program Files\Alwil Software\Avast5\AvastSvc.exe.)

[TheScorpion]

Thanks for the reply Olddog - looks like you are right!
I'd just been prowling around Comodo and discovered that under Firewall/ Network Security Policy there was a 'Block' assigned to one of the Avast entries. I've just "Allowed" this and the updates have gone through ok. Don't know how this changed, but it looks like the problem has been solved at last!