the reason you are getting this message is you probably incorrectly uninstalled Yahoo! pager or messenger.
http://www.faqts.com/knowledge_base/view.phtml/aid/13699/fid/493
http://www.google.com/search?sourceid=navclient&q=missing+idle%2Edll
I am getting " a required .dll file, idle.dll was not found each time I turn on my computor. I click OK and go on with everything except loading Yahoo Messenger. Then the same message comes up and under it is "could not execute the external program, c:\program~1\yahoo!messenger~1|ypager.exe" I can find msidle.dll in the computor's files. Also before this happened i had uninstalled Yahoo Messenger but i don't know if this problem started then. Any help would be appreciated.

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