This has all the signs of a bug in a recent Avast prog update.
I've also used Avast for some time now and a variety of red 'cannot connect to update' error messages have been appearing for the last couple of weeks or so. I've also been asked by two other Avast users about this problem over the same time period.
Today my error message was: 'Cannot connect to download36.avast.com'
It's now happening on both of my recently re-installed XPpro partitions, and over the last few days I have re-installed Avast 3 times on both - without any resolution. There are no blocked entries in my Firewall. Manual updates of both prog and vps work normally.
I noticed elsewhere that the New Comodo version is having the finger pointed at it. That could be so, but I am still using the same old Comodo v2.4.18.184 that has worked perfectly with Avast for a year. (I always avoid new versions of software until they have been fully shaken down!)
Interestingly, the problem is not happening on my old Win98se box, which is updating perfectly.
Thinking caps on boys?

Regards, Bill