Go to start and in the search box type msconfig, go in to start up and untick the box. If you have C Cleaner you can also do it in the tools section of that. Highlight it in the start up section and click disable on the right. You should also be able to do it from within the options section of Yahoo messenger itself, but I've never used it so aren't sure where to find it. I'd bet 100 to 1 it's in options or preferences though.