Most network administrators consider it a best practice to perform any necessary maintenance tasks after hours whenever possible. However, the Internet has blurred the line between working and nonworking hours, and you can't be certain that no one is working on the network just because it's after 6 P.M.
To help you out, Windows NT 4 shipped with Server Manager (Srvmgr.exe), a utility that tells you how many users are using resources and allows you to notify them. You can find Srvmgr.exe in the Administrative Tools folder.
When you open the utility, you'll see a list of computers on the network. Double-clicking a computer opens a dialog box with additional options.
Once you see how many users are accessing computer resources, you can notify them of any actions you need to take. For example, if you're restarting the machine, make sure you notify users so they have enough time to save their data. To do so, select the computer, and go to Computer | Send Message in Server Manager. Enter the message, and send it to all users who access resources on the computer.