If you're getting those while online and using Internet Explorer, ignore them. They're designed to scare you into paying for rubbish software, and sometimes contain malware themselves.
You could do two things. One, run scans with all the usual antimalware: antivirus, Windows Defender, SA Squared, Spybot, Adaware, Superantispyware just to make sure there is nothing out of order on your machine; and you should do that regularly anyway. And second, stop using Internet Explorer and switch to Firefox click here with the Adblock click here, Adblock Plus click here and Filterset.G updater click here extensions. That way you will never see those popups, or indeed any other advert, again.