By default, you are able to run only one instance of Yahoo! Messenger on a computer (under a single user account). Although you can create multiple user accounts to log in to several Yahoo! Messenger accounts simultaneously from a single computer, it involves lot of time and effort in switching users.

Adding a simple entry to the Windows registry will let you run multiple instances of Yahoo! Messenger simultaneously and sign in to as many accounts as you may have, all from a single user account on the same computer.

1. Click Start
   
2. Click Run
   
3. Type regedit in the Open input box and hit ENTER key
   
4. Navigate to:
   HKEY_CURRENT_USER\Software\Yahoo\Pager\Test
   
5. Click New on the Edit menu, and then click DWORD Value
   
6. Type Plural, and hit the ENTER key to name the entry.
   
7. In the right pane, right-click Plural, and then click Modify.
   
8. In the Edit DWORD Value dialog box, type 1 under Value data.

Now run Yahoo! Messenger as usual. Every time you click/double click the Yahoo! Messenger icon in your Start Menu, on your Desktop or your Quick Launch a new instance of Yahoo! Messenger will be started. You can now sign in to multiple Yahoo! Messenger accounts and be notified of any new email you have on any of your Yahoo! accounts.