If you’ve installed Yahoo Messenger on your Computer through Online installer, instead of offline Installer, you should know that Yahoo Messenger installs the Yahoo AutoUpdater service, which, checks for Messenger updates and notifies you about the same. As a result, you’ll see YahooAUService.exe process running in the Task Manager (this process doesn’t appear when Yahoo Messenger was installed offline).
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Disabling YahooAUService.exe or uninstalling Yahoo AutoUpdater
To disable this process from running in your Task Manager, you need to uninstall Yahoo AutoUpdater component installed on your Computer.
For that, navigate to “C:\Program Files\Yahoo\SoftwareUpdate” directory, and double click on the Uninst_AutoUpdater and click on the Uninstall button. The Yahoo software Update will be uninstalled and this process disappears from the task manager and won’t appear again.
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No help removing from Win7
As said in the post, you need to remove Yahoo Autoupdater component.