This could be a few things:
1. DNS issues. Can you open a command prompt and try pinging the website of interest? For example: ping www.google.com. Do you get any results? If the name is resolved, then there could be a browser or firewall issue for HTTP traffic.
2. Open a command window and telnet to port 80 on the website of interest. Does it connect? If so, then there is a browser problem (proxy server configuration?).
3. Is there a firewall blocking HTTP traffic?
4. Does the computer have another browser (Firefox, Chrome) to try accessing the same website?