In the Latest Version of Yahoo! Messenger (v.8), Yahoo! has taken a page out of Microsoft’s Book by placing annoying flash ads at the bottom advertising Pc-To-Pc Calling, personally i find the ads VERY Annoying, there is a easy way to remove the ads, here’s how to do it:
Firstly, You need to know your version of YIM, To know this, simply go to Help> About Yahoo Messenger and it will show the version installed.
Now, Make sure Yahoo! Messenger is NOT Running, not even in the tasbkar; if you see a grey emoicon on the taskbar, right click on it and select ‘Exit’
Now Depending on your version of Yahoo! Messenger, Download one of the .Zip Files Attached below, If you’re using a messenger version which is lower than v 8.0.0.683 then download the ‘No Yahoo Ads Ver8.0.0.683 or lower’ ZIP File, Similarly If you’re version is 8.0.0.701 or above Download the ZIP Named 'No Yahoo Ads Ver8.0.0.701 or above’Extract the contents, it contains a clickable .bat file, just Double click on it and you’re done! the .bat file simply modifies HKEY_CURRENT_USER\Software\ Yahoo\pager\YUrl and replaces several values with an Asterisk * , The Values it modifies are:
Messenger Ad
Webcam Upload Ad
Webcam Viewer Ad
Webcam Viewer Ad Big
Webcam Viewer Ad Medium
Change Room Banner
Conf Adurl
Chat Adurl
After applying it simply Log-into messenger like you normally do but this time, there are no pesky ads!
NOTE: After applying this tweak, you WON’T be able to Enter ChatRooms or use Audibles, If you reinstall Yahoo! Messenger, then you have to repeat the steps mentioned above.
Source: Google Operating System @Blogger
Hope this helped 
Please Correct Me if i made a mistake somewhere, i am new here 