1) select Start->Run
2) click Browse button
3) find unwise.exe in Yahoo! install folder (usually C:\Program Files\Yahoo!\Messenger\unwise.exe), click OK in 'Browse' window
4) after that you should see in Run window the following:
"C:\Program Files\Yahoo!\Messenger\unwise.exe"
5) edit this line, so it looks like:
"C:\Program Files\Yahoo!\Messenger\unwise.exe" install.log
(after "the path" put the space and type install.log)
6) click OK in Run window
That should uninstall yahoo messenger.