I was called today by a client today with a strange problem: yahoo messenger would error as soon as she wanted to start a new chat. The error quoted mshtml.dll as the source of the problems. Reinstalling Yahoo Messenger (which I previously suggested over the phone did not help).
I don’t know exactly how this problem started but the fix is the following:
go to the Microsoft Website and download 912945 (http://support.microsoft.com/kb/912945/ ) Internet Explorer ActiveX update.
Please note that this is a verified download (you need wga)
I installed the patch and the problem went away. Yahoo Messenger now works fine.