Browsing internet

Hi,
I have a windows 2000 Prof.computer, with what appears to be an browsing internet problem.I’m able to browse the internet w/no problems but there are certain websites I’m unable to browse ex: msnbc.com is one site I’m unable to bring up and there are some others. I do go through a proxy with a very fast response… I currently have I.E. 6.0. . I tried to reinstall ie6.0 with the same results.I tried changing the security settings… Any help would be appreciated.