Sign out from Yahoo Messenger, navigate to Start, type cmd and open the utility using Administrator rights. Type the following commands and press Enter after each one:
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rd "%AppData%\Yahoo!\" /s /q
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rd "%ProgramFiles%\Yahoo!\Messenger\Cache\" /s /q
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rd "%ProgramFiles(x86)%\Yahoo!\Messenger\Cache\" /s /q rd
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"%LocalAppData%\VirtualStore\Program Files\Yahoo!\Messenger\Cache" /s /q
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rd "%LocalAppData%\VirtualStore\Program Files (x86)\Yahoo!\Messenger\Cache" /s /q
After that, close Command Prompt and reboot your computer.