More details please. "Do not connect" is pretty vague. What kind of error do you get? Does Safari (or whatever you're browsing with) say it could not find the site, or does it say something else? Does it do this with all browsers?
Describe your network, from the telephone cord into the DSL modem all the way to each computer, including model number for the modem, which AirPort base you have, and what computers you have as well. Especially explain how your PPPoE connection is handled. And most especially, who is your ISP? (They are far from perfect, even the best ones, and sometimes they have problems that can show up this way. Remember, FTP is much more forgiving of momentary disconnects than either a browser or a mail app.)
Finally, is there anyone else around you with a wireless network? Does your AirPort indicator ever offer to connect you to some network other than your own?