why do i get "This webpage is not available." for only some sites?

i am running windows XP w/ SP3 and in google chrome i get the message "webpage not available" the same pages are not available using IE 8 as well. funny thing is most sites work, but not windows update, and other misc. spybot update and windows defender, i have the latest updates on my ESET internet security and no threats were found on a "in depth" scan.

so why can i not connect to some sites, while others work?

I access the internet, by way of a hard wired connection to "2701HG-B Gateway"