Do you want to run multiple instance of yahoo messenger with different users logged in on a same computer? It can be done easily.
First open regedit. Click on Start button and type regedit in the search box. Then click on regedit in the search results. Navigate to HKEY_CURRENT_USER > Software > Yahoo > pager > Test. Here, right click on the right pane, and create a new DWORD with name Plural and change its value to 1.
Close regedit, and restart your yahoo messenger. That’s it. Now, to open another instance of yahoo messenger, when one is already open, simply click on the yahoo messenger shortcut on desktop again.