Can't access many websties -- tried everythiing

Running Win 7 on an HP desktop, Norton AV. I cannot access many websites, such as facebook.com, nytimes.com. I can access google.com, msn.com. On inaccessible sites, hangs forever, sometimes at an interior url for the site. Here's things I have tried or checked:

• multiple browsers: firefox, chrome, IE, all latest versions -- all fail the same way
  
• same computer, different service provider in different city -- works fine (this is the most frustrating)
• different computer on same service provider & modem -- works fine (obviously)
  
• ping all the inaccessible sites -- fine, can ping them, no packets lost (so not modem issues either)
  
• set up OpenDNS in IPv4 -- did not help (had a lot of hope for this; could I have done it wrong?)

I have not looked into proxy server issues and do not know how.

Any ideas? I am doing this for my sister in another town, and since I can't get it to fail when I bring the box to my own office, it is very frustrating. (Anyone wanna but a nice new HP desktop?) :sad::sad::sad:

Thanks. --David