Try this free detection and removal software:
https://www.malwarebytes.org/antimalware/

I assume you're online when you get these popups or a new page is opened up? Sleazy sites... like those that have free movies, tend to make their money with such ads... and they're designed to scare... SHIT!! the site you were just at infected you PC!! Click link below to start scan!!!! Some of these ads are designed to look like official notices from Apple or MS.

I don't know what browser you're using... but don't they all now have popup blockers? Firefox has a number of free plugins that block ads... like Ad Block Plus. If you're visiting sites that throw up a barrage of windows... try No Script. It might break some of the sites you use... because it blocks/limits javascript. But you can white list sites you trust, or limit the scripts running on a site you don't. You could also try a IP blocker like Peer Block that has different lists... such as for ad sites, and it just prevents any connection to them even when you're not using a browser.

If the popups are appearing outside of the browser... then that's a bigger warning sign. While some free programs have nags... the free version of Avast has nag popups... if they're for sites that have nothing to do with a free program... than you may have adware on your PC. There are free programs that can scan your PC... Malwarebytes is one. AdAware is another.