Normally, when you start your PC, under Windows 2000, XP and Vista systems, there is a window which appears asking you to choose which user will start the session. It will then load the predefined configuration for that user. But don`t think that it produces just one session start.
Logon Sessions is a simple tool that lets you know exactly how many sessions are active in your PC at any time. Also, if you specify the -p option, it will show you the processes running in each of the open sessions. It works under the MS-DOS system, via the command logonsessions [-p].