|See this very recent post.|
While "phil's" suggestion may get rid of it, I advise you run the cleaner apps I suggest in that post. Popups are usually caused by nuisance items either downloaed and installed (unintentionally) when installing some software off the web. Also some web content may also drop them into your system as you navigate around it.
Whenever you download and install software (utilities etc. especially) from the web, always use the manual/custom option; never the default/automatic option. Watch for and uncheck "all" pre checked boxes, other than the one for the utility etc. you actually want. This way you redice the chances of acquiring all manner nuisances, pests etc.; many of which are real pain to eradicate.