My WIFE has a HP9000 laptop running Windows XP.   She is using WiFi connected to a Netgear wirelss  router  in my home network.   My base computer is a desktop running Windows 7 64 bitand is connected to the router via ethernet cable.   For a couple of years now this system has been working satisfactorily....but...    

PROBLEM:   When she clicks on internet explorer, the laptop connects to the internet and tries to  bring up Yahoo or Google home PAGE  a message appears:   "Internet explorer has ENCOUNTERED a problem with an add-on and needs to close". 

the name of the add-on is       yt.dll

If I check my internet connections it appears to have disconnected.
Can any one advise me of  the problem??

NOTE:    Sometimes she can get google but cannot access any of the web sites in her favorites list.
OOOPS my bad.  Her machine is running VISTA. Uninstall the Yahoo Toolbar.Thanks for answering.     OKAY I un-installed Yahoo tool bar which did change the way it acts but did not fix the problem.  Still cannot access any of the sites on favorites list.