Switch user option no longer appears on Vista menu

Once fast-user switching is disabled in Vista, (Stand-Alone systems not on a Domain) there is no simple way to re-enable it.

Resolution :

Start >> Start Search >> regedit OR Start >> Start Search >> run >> regedit

HKEY_LOCAL_MACHINE >> SOFTWARE >> Microsoft >> Windows >> CurrentVersion >> Policies >> System
Look for “HideFastUserSwitching” on the right hand side (If the “HideFastUserSwitching” option is missing, create a  DWORD (32-bit) Value named HideFastUserSwitching)
Change the value from 1 to 0
Restart the computer.