you didn't mention your browser (IE,FF,Opera), but go to Tools:Options
and empty the cache; quit the browser.
get a cmd prompt; enter
ping $yourISP {eg earthlink.net}
you must get some timing info and not get a series of
timeouts
launch your browser and type in the url HTTP://www.$yourISP/
you must get the home page of the ISP.
{I always use my ISP as the browser default home page, as it tells me if I
have connectivity to at least my ISP

} When this works and other sites
don't, then it's a problem upstream from your ISP.
Sometime I find it helpfull to get a cmd prompt and use
ping -n 1000 -w 5000 $yourISP
this gets the DNS charged-up and running and helps avoid DNS lookup errors,
which always showup as 'Page Not Found'