Yahoo Messenger

you can go to control panel-> add/remove program-> and look for all the Yahoo Messenger related item and remove them ...
if it still cannot be removed, use HIJACK THIS to remove it from running in windows completely.
Download it first and install, it can help you to remove stuff from registry and startup.
http://www.trendsecure.com/portal/en-US/threat_analytics/hijackthis.php
and once it is not running, you can wipe out the whole
C:\Program Files\Yahoo!\Messenger
directory completely.
hope all help