Yahoo is discontinuing its latest version of Yahoo Messenger from Windows Vista Operating System. Some months back Yahoo has launched Yahoo Messenger for Windows Vista. According to Yahoo because they want to focus on delivering one Windows experience that is optimized for all Windows users.
As I see Yahoo is just want to develop Yahoo messenger that works consistently well in all windows operating systems not just in Windows Vista. Currently Yahoo Messenger application is based on Windows Presentation Foundation and Yahoo will continue with this technology in future version of Yahoo Messengers also.
If you’re using Yahoo! Messenger for Vista, you can continue to use it however we will not add any more features or provide product support. To enjoy full Yahoo! Messenger functionality including photo sharing, webcam and chat rooms, we recommend downloading and installing the latest version of Yahoo! Messenger 9.0. This latest version of Yahoo Messenger is also available in Local Languages like in India it is available in Hindi.
Source: Yahoo! Messenger for Vista version is no longer available
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Yahoo Messenger for Vista was a bad idea. I hated it because it was made to be incompatible with XP for no reason (it was possible to run it on Xp with a simple modification).
It looked better, but I found its user interface to be more inconvinient.