try unistalling and reinstalling yahoo.
this will not affect your contacts but you will lose the message history for all your contacts.
if you would like to keep the history then do the following
go to file manager
program files > yahoo > messenger > profiles > (your yahoo ID) > archive > then right click on archive and then copy and paste it onto your desktop before unistalling.
once reinstalled then put the folder back
Andrew Wyatt
August 2007