ICQ 8

If you're in single window mode, your contact list will be attached to the conversation window, allowing you to more quickly switch to conversations with new users. All of the contacts with whom you are currently having conversations will be gathered in a list in the "Active conversations" section. To switch between one conversation to another, click the name of the desired contact in the list. 

To disable single window mode and go to the standard window with tabs, click the "Separate windows" icon located above your avatar.

In the tabbed window mode all of the contacts with whom you are currently having conversations will be gathered in a single window. To switch from one conversation to another, simply click on the tab with the name of the desired contact.

To return to the single window mode, click the  "Merge windows" icon located above the active conversation tabs.