Why Does My Yahoo Messenger Say CoCreateInstance for the MyYahoo Dll Failed?

Function of CoCreateInstance and MyYahoo.dll
Unless you're a Microsoft Visual Basic programmer, you probably have no real need to understand the details of how the CoCreateInstance function works on your computer. Nevertheless, in short, the function writes parameter values for a program to the Windows Registry and creates COM objects that allow other applications to work. MyYahoo.dll is a dynamic link library file, or DLL, needed by Yahoo Messenger itself. If the CoCreateInstance function fails to write to the registry or create a COM object, MyYahoo.dll can't initialize and Yahoo Messenger will not launch.
Common Causes of Errors
In most cases, CoCreateInstance or MyYahoo.dll errors are the result of a faulty installation, which can occur if you downloaded a corrupt setup file, failed to close other applications before installing Yahoo Messenger or your anti-virus application blocked certain files during setup. Additionally, if you upgraded from one Windows version to newer one -- and didn't perform a clean install from scratch -- changes to the registry might cause the errors as well.
Starting Over
More often than not, simply reinstalling Yahoo Messenger fixes most CoCreateInstance or MyYahoo.dll errors. However, before doing so, you should remove the current version of Yahoo Messenger from your system completely by using the “Add/Remove Programs” option in Control Panel. Before reinstalling Yahoo Messenger, download the latest version of the application from the Yahoo.com website, close all applications -- especially other IM programs or chat client -- and disable your antivirus software temporarily. After you reinstall Yahoo Messenger, restart your computer and attempt to launch the software. It should function as intended.
Other Options
If uninstalling and reinstalling Yahoo Messenger doesn't fix your CoCreateInstance or MyYahoo.dll errors, you probably have problems in your Windows Registry. Download a free registry repair/defrag tool such as GlarySoft Registry Cleaner, Eusing Free Registry Defrag or CCleaner and run it on your computer. After cleaning or repairing the registry, uninstall Yahoo Messenger and reinstall it. If that fails, your only option is to reinstall your possibly corrupted Windows operating system. If you do reinstall Windows, perform a clean install without upgrading from a previous version.