As Tech says this could indicate a problem with your DNS, that was one of the reasons I threw in the IP address link, another reason is the windows hosts file could be blocking avast.com. So it might be worth while checking that out. C:\Windows\HOSTS this is a file (don't confuse with a folder) that has no file extension.
It can be opened and edited with a text editor like notepad, once opened check there are no entries for avast.com with 127.0.0.1 in the line, if so report your findings. Some malware does this to divert your attempts to get to an antivirus site to your computer, this way you can't get to AV sites to have your system scanned, updated, etc. to remove malware.
It may be nothing just sluggish connections by you ISP, but it is worth checking out.