Eliminating Popups

Hi
I have been asked to clear popups from several systems just recently. Usually these appear in a window in text form. Mainly advertising and not always porn!!
Is there a way to stop them appearing other than by downloading a program such as “StopZilla”? Some customers seem to resent having to pay for a program in addition to my charges.
Whereabouts in Windows are these popups usually installed? I have used “Stopzilla” on my system for a trial period and find it works well but will it stop prexisting popups?
Is there likely to be a reference to them in MSConfig?