Probably the easiest way out of this one is to go to Add and Remove programs in the Control Panel and remove anything Yahoo.
If this doesn't do it then go "Start" "Run" and type "msconfig" and press enter. Look for the "startup" tab (top right) and then uncheck any box that has Yahoo in it. You might need to grab the column width handles at the top of the page so you can see all the program name. All this is assuming you're on XP. if not, ignore all of this paragraph!
Good luck!