1. Open the registry editor by going to: start> run> and type "regedit" w/o the quotes.
2. Navigate to HKEY_CURRENT_USER\software\yahoo\pager\test
3. Right click the right panel window, choose new> DWORD value> and type "Plural" w/o the quotes.
4. Modify the Plural entry by double clicking it, and enter value data "1" w/o the quotes.
5. Click Ok and close the Registry editor.
That's it! You can run multiple Yahoo! Messenger (any version as far as I know) app's and can log-in as many users in 1 pc.