Erasing Porn

Porn websites are continually popping up on a Win 98 PC with a cable connnection. I have erased a number of .exe files but after a few days there are sites in it’s own favorites folder. How do I eraase and check for porn on this computer? He did not have a firewall installed and says he did not access any porn at all. Maybe the teens have accessed and possibly downloaded a dialer. Not sure, need suggestions.