1) Hit Start > Run and fire up regedit
2) Look for the section:
HKEY_CURRENT_USER Software Microsoft MSNMessenger PerPassportSettings
3) Open all the sections consisting out of a number combination.
4) Select the DWORD DisableTabs (if it is not present, create it by right-clicking, selecting New and then DWORD-value).
5) Double-click it and change its value to 1.
6) Repeat this step for all number combinations.
7) Close and restart MSN Messenger.