File server with windows 2003: how to log off ?

Hello,

in my company we have a file server runing with windows 2003.

All the employe take the file on this server. To access on it they must use their own login and password. Some folder a accessible only by few people.

My question: after log on, how to disconnect to log on with an other login and password.
Even if i logoff or restart the computers it change nothing !

Any idea ??

Thanks a lot

Pierre