Add-on toolbars in general are not good news, as many of them are loaded with spyware. I can't imagine Yahoo being the exception to that rule, so if it were me, I would bin it. If you want a pop up stopper, try the Google Toolbar, which I don't believe contains spyware, but I'm not certain. There used to be two versions of it, a standard and an 'advanced', the 'advanced' version did contain a tracker, but there only seems to be one version now.
Better still, have you considered moving to an alternative browser? Many of us on PC Pals use Mozilla, which contains a built in Pop Up Stopper, plus many other useful tools, not included in IE. It's also less prone to the many security glitches found in IE of late. We have two Mozilla threads at the top of our
General Software Help section.