im using a adsl connection for internet. when i try to go microsoft.com, avast.com, avg.com.......it says web address cannot be found. i tried on mozill, ie and google chrome as well. bt the result is same. other web sites work perfectly without any issue. im using a zte zxdsl 852 modem and avast 4.7 professional virus guard.
assuming your urls you type are correct, and assuming you aren't hitting the servers just as they happen to be down (which is highly unlikely), i would point my finger at some firewall.
whether this is implemented in the software or hardware i am not sure, but looking at avast would be my first action. perhaps try disabling it for a test? (if you are paranoid about viruses, just for a few seconds)
from my experience "protective" software is only good for slowing down your pc.