I was actually thinking of starting one of these threads myself.
It was about three or four days ago. I hit PGC and saw a pop-up starting to launch, and suddenly it expanded to cover my entire monitor, including the taskbar, and left no option to close it.
It said something to the effect of "If you're capable of seeing this message, and it's covering your taskbar, then odds are you have spyware on your system. Please click on this link to download and install our anti-spyware programs." I'm not that stupid, so I just nuked Internet Explorer entirely with a quick control-alt-delete.
When I opened IE again later in the day, that same stupid pop-up got in my face. I control-alt-deleted just the pop-up, and saw that my home page had been changed. This website looked almost the same, but it said something to the effect of "If you're tired of people hijacking your homepage, then you probably have spyware on your system. Please click on this link to download and install our anti-spyware programs."
I changed my homepage back to normal, and for the heck of it I ran Lavasoft AdAware (which I already had installed, and use on occasion), and performed a full Norton virus-scan. I got the clean bills of health that I always get.
I'm guessing that one of your advertisers is (or was, or was hacked into) using some simple internet parlour tricks to try and scare PGC users into thinking they need whatever new software these guys are selling. I don't think it has anything to do with spyware.
I suppose it wouldn't hurt to have a more secure internet browser, but I've never seen the need for one, as I have pretty good judgement (usually), and don't frequent the kind of sites that make this sort of thing a problem. This one harmlessly annoying incedent actually puts PGC slightly ahead of anywhere else I go on the "problem meter". And I have some respect for the advertisers who keep sites like PGC alive, so I don't really want to turn on a pop-up blocker. And PGC actually has the best, most interesting advertisements (since they're almost all videogame-related) of all the sites I know of.
Anyways, I'm not sure if this is still a problem, since I just hopped straight into the forums today without hitting the main site, nor do I know how to fix it (have a talk with whoever gets you your ads, I guess?), but I figured I should let you know that it's happened to more than one person.