roger,
There are some sites that will use a javascript that will automatically change the home page of your browser and also insert sites into your favorites. This script is in common use in the internet underworld (warez, porn, and other such sites).
Many of these scripts are recognized as viruses now. In fact I wrote a script that was similar and it was recognized as a virus when I tested it. So most of those scripts should fail against an updated anti virus program now.
gezzy,
As to preventing it:
a) someone downloaded this IMO, it's possible that they thought it was something else but it's highly unlikely that this program got there without help from a user of your PC that was tricked into downloading it.
b) In my limited experience with helping people with porn dialers I have never seen or heard of one that didn't come from a porn site. It's entirely possible but the general MO for these dialers is to fake that it's porn, the user thinks a movie is bein downloaded but it's a dialer.
Now, it's 100% possible that this did not come from a porn site but through some other trickery.
But it is HIGHLY unlikely that it got there without someone pressing "ok" to a download. The download could have been called something else and have tricked the user into downloading it but to get there without such steps requires a set of rare circumstances.
I don't want to get someone who has access to your PC in trouble, it is quite likely that the download was misrepresented by the site it came from. But what I think (I can be wrong) is that someone let the program in. And to stop it more vigilance on the part of the users of the PC is required.
As to a firewall, depending on the firewall it will stop the outbound dialing but it will do nothing to stop the installation of these programs as long as someone can be tricked into downloading them.
But firewalls are good to use. The best free one is Zone Alarm
http://www.zonelabs.com/ but I personally wouldn't be caught dead using it (for personal and picky reasons, the software is ok).
I use one that is built into Windows XP and then employ some other tricks to stay safe (these tricks require remte computers and are not practical so I won't suggest them to others).
Larry,
Porn dialers can be a perfectly legal piece of software and if anti virus companies added them to ther virus definitions they'd be sued. Now some dialers user virus like programs to install themselves and those mioght be flagged but the overwhelming majority of porn dialers are legal and the trick is in getting the user to download it thinking it is something else.