Parasites/Bugs/Virus are the usual cause for this. Run the tools listed below, be sure to update the programs before running them.
I somehow have aquired this toolbar on my Internet Explorer. I don't know how I got this thing. When I go into toolbar options and uncheck it, Intelligent Explorer toolbar is back when IE is restarted. Ever since It appeared, pop-ups have been getting through all my blockers. Anyone know how to get rid of this thing?