Hi,
I’ve tried to update my avast internet securities a lot of times, unsuccessfully… :-[ I’ve also tried clean install but the problem persist.
when I try to update virus definitions it sends every time unknown error. Even reinstalling my OS I did. It is Windows 8.1.
I’m already tired doing that.
Please, I need instructions beyond reinstalling AIS
Not working:
I see that the problem is the downloading part.
Maybe it helps
See if this fixes the problem:
http://www.thewindowsclub.com/how-to-change-dns-settings-in-windows-7-vista
To use Google: 8.8.8.8 and 8.8.4.4
Thanks! That worked out! 
But now it worries me. In past I’ve already started having problems with DNS stuff, and I had to do a lot of tricks to be online (like going to prompt command flushdns etc).
Then I had to reinstall OS cause that was unsustainable, and then I had many months of peace.
Probably this is something structural about windows or virus related.
If that is virus related, witch routine should I do to find some hiden ones? I’m afraid about having a source of virus in some device like routers… :-\
You’re welcome. Flushing your DNS isn’t trickery. It’s called housekeeping similar to cleaning up temp. files etc.
I on a regular basis, disconnect my router and restart it. I do the same with my computers including removing the battery from the laptops.
I look at it as preventative maintenance.
Flushdns on a monthly basis is not big deal, but more than 5 times a day is a big deal. I hope that that stays in the past…
I believe your DNS problem is more likely to be your previous DNS server wasn’t capable of handling IPv6 IP addresses, but the Google DNS servers are (I believe) able to handle IPv6 addresses.
IPv6 Test
IPv6 - Wikipedia
HowIPv6 Works: IPv6 - technet.microsoft.com
I apreciate your help! I’ll add this to my toolkit, thank you! 
You’re welcome.