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What happens when you try to connect to avast.com, what error messages, etc. ?

HOSTS file redirect - 127.0.0.1 check your HOSTS file using notepad or a text editor of your choice, C:\WINDOWS\system32\drivers\etc\hosts or do a search for HOSTS to find it if not there. http://en.wikipedia.org/wiki/Hosts_file

It may be that there is an entry for avast.com placed there by malware trying to stop you getting to anti-virus web sites. Is there an entry for avast.com in the hosts file ?
If so please post the contents (copy and paste) of the file here.