Hey harpeter.
It is a pretty common scam nowadays for these pop-ups to show up and display that you have all manner of bad stuff and then tell you that the only way you can remove it is by buying their stuff . . . it's appropriately called "scrareware".
You likely got this from Limewire. I'm not suggesting necessarily that you don't download music from Limewire, but it's a hotbed for malware so if you're going to use it, make sure your virus protection is up to date and run an on demand scan after you download any music from there.
Now before Limewire fans jump all over me and protest that "I've been using Limewire ever since it came out and I've never gotten an infection", I agree. There's plenty of people who have never gotten bad stuff from Limewire. But there's plenty that have too . . . so just be extra safe if you choose to go there.
Whenever a pop up presents a link for a download, DO NOT click on it, no matter how badly some may say your computer is infected. If you're scared that it may be infected, run an on demand scan with the LEGITIMATE antivirus software that you have on your machine already.
One of the particularly popular scams lately is a pop up that will scare you by telling you that you have 100's of registry errors, and the only way to "clean" them is to click on their "Registry Cleaner" link. I recently saw one that claimed the user had over a thousand registry "errors". If you had that many registry errors and they were serious (which the scam implies, if not outright saying so), then your machine wouldn't even be running.
Before Rick jumps in here and corrects me, I'm not saying that you can't have 1000 registry . . . trivial defects. But they are not serious errors that will cause your machine to malfunction. They are things like links pointing to non-existent help files or missing desktop icons, which is pretty trivial.
Finally, these scams will actually give you more infections if you download them, NOT remove infections. Plus, if you buy that crap then the scammers will likely have enough of your personal information to steal your identity or make fraudulent credit card charges.
In fact, whereas infections used to be something that bored malware writers used to do just for "kicks", infections nowadays are written with an eye toward getting enough of your personal information so that these scoundrels can steal your identity. It's big business now.
So don't ever click on a download link in a pop up . . . nor should you click on links in emails or IM's. More than one person has been infected by clicking on a link in an IM that says something like "Click on this link to see a cool pic."