There was a problem with my ADSL modem and it was replaced by the provider. Its connection type is pppoe. Since then, I can access all websites normally when I use Windows, but when I use Linux (Ubuntu Dapper), either on the same machine (a dual boot machine), or from another machine running the same OS, some websites are available, whereas with others, the browser just waits for some sites to load forever, while some sites are accessible. I CAN succesfully ping even the ones I can't access. The inaccessible ones aren't accessible via browsers, e-mail clients or IM clients, so this is not a browser problem. In rare cases, webpages load so far as to change the title in the browser or display a few first words.

Again, the problem is the same with two diffrent machines running Ubuntu Dapper, which had performed perfectly before the modem change. But when I run windows without changing anything in the modem, everything is accessible.

Any idea what the problem might be and how to solve it?

Disable IPv6 and try again. This will cause the symptoms you describe.

Thank you very much. Could you elaborate? My technical understanding is very limited, and I do not know what IPv6 is and how to disable it.