I don't think Yahoo Messenger uses 100% CPU. Check whether you can any other services installed (like file indexing service etc., ), or your System may be infected with a virus. Use Norton antivirus and scan all the files in the PC.
You can also stop the services that you don't need. If you are using XP Pro,
Click Start button -> Run. Type 'services.msc' and stop the services that are
not required. Also disable the services that are not needed. I think this can surely help.