I'm sure if you've used Yahoo! Messenger you've noticed the adverts built in to the chat and webcam services. Here's a simple fix and it works great. Simply download http://www.freddys-utilities.co.uk/noads.exe & http://www.freddys-utilities.co.uk/noads.jpg and save them to a directory. Run the noads.exe file and then start up Yahoo! Don't delete the files after you save them or it'll stop working.
Don't worry about the .exe extension, it just points Yahoo to that tiny noads.jpg and replaces the ads with it.