Sent to you by Vihang Shah via Google Reader:
Then new window will open.
Goto HKEY_CURRENT_USER -> Software -> Yahoo -> Pager -> View
Inside the 'View' key there are a several other keys… go through each one and edit the 'banner url' string to your liking. It even works if the string is left blank (this causes it to look as if ads were never even implemented)!
Restart Yahoo! IM for the effect to take hold.
This is great for Yahoo! IM 8 users who don't want to be bothered with ads.