It has been found that Yahoo Multi Software can be downloaded from various sites to run multiple yahoo ids at a same time.
But I have a simple way to do it via the Registry.
As it is the fact that every information regarding our pc is registered in Registry. And most of us know what registry actually is and what can be done from a simple modification in its registered information.

For running multiple yahoo windows:

Add a Reg_Dword value "Plural" with data "1" in the registry location: "HKEY_CURRENT_USER\Software\yahoo\pager\test"

Or simply you can type the following code in DOS or even make a batch file and execute it..

reg add "HKCU\Software\Yahoo\pager\Test" /t reg_dword /v "Plural" /d "1" /f

thanks....