PUSHy Browser has mind of it’s own

When running IE5 under Windows 98 Second Edition, on a brand new HP Pavilion Desktop PC, a popup browser window appears every 30 seconds with various advertising, no matter what site I choose to surf in my main browser window. Is this a registry setting or something for pushing data to my browser. Or is it a shortcut of some kind, hidden somewhere on my harddrive or in my browser settings? I don’t like it, and really want it gone. All answers appreciated.