A while back I had a similar experience when trying to research a medical issue....I clicked on one of the results from my Google search and was taken into a continual loop of pages relating to viagara, sex toys, blow up dolls and porn products. Nothing I did would break the loop, it was just one page of trash after another....and worse still, I could NOT close my browser. Every time I closed IE, it would instantly re-open and continue the same loop....and not one page related to any kind of medical advice/info that I requested, it was all crappola.
In frustration, I did a forced reboot of my PC, but the moment the OS had fully loaded, lo and behold, there was IE restarting in the same unstoppable loop. I noticed there was a browser add-on that wasn't there before and tried to disable it, but all I got was a message saying it could not be disabled due to being currently in use....thus a restart in safe mode (without networking) was necessary to access and disable it.
The add-on was just a bunch of random numbers and letters, and it was a while ago so I can't remember them now, but Adaware, Spybot and other spyware/malware proggies wouldn't touch it. Also, disabling it was only a temporary measure...doing the same or any other Google search reactivated it, so a system reformat became necessary to completely remove it.
I don't know if this particular nasty is still around because a report was submitted to MS, but it was nigh impossible to get rid of, so hopefully it has since been eliminated/counter-acted and will bother nobody else.
Strangely enough, since abandoning Google products & services entirely, switching to Firefox and Yahoo for searches, I have not had a problem with such add-ons...nor have I been inundated with porn/viagara related spam since ditching Google.
Funny thing though, I recently received a couple of emails from someone "desperately seeking a partner"

Might see if I can get Hotmail to attach my pic to my email addy....that'll stop 'em dead in their tracks.