It appears that your browser has been compromised, infected with malware. To test this idea, try opening HN in a different browser, or uninstall and reinstall your present browser, making sure to delete absolutely everything before reinstalling.
To me it looks like you have some kind of spyware on your computer. Check your local hosts file for unwanted entries:<p><a href="http://en.wikipedia.org/wiki/Hosts_(file)" rel="nofollow">http://en.wikipedia.org/wiki/Hosts_(file)</a><p>Also have a look at the source code of the HN page if there is anything which might explain this behaviour. Maybe it is running inside a frame site which opens the ads? There are so many possibilities ...<p>If there really is some malware on your system I recommend the procedure known as <i>Nuke it from Orbit</i>: Wipe everything clean, install a clean operating system, restore important things from a backup. And change all your passwords from a clean system.
localhosts: check<p>i suspect it's the free WiFis that require you to comply with their terms and conditions. They probably catch the traffic and inject text ads into the HTML, which arguably is quite clever and malicious.<p>i'll get more data to confirm this