Broadcasting a message to all your users

Net send is a very good idea, until SP2 which disabled it by default. It can be used to warn users; example (run command first):
net send * servers will be shutting down in 5 minutes.

Documentation Microsoft Windows XP - Net send
Troubleshooting The NET SEND command may not work correctly on a computer that is running Windows XP Service Pack 2 because messenger service must be running, but is set to disabled on SP2, so

This problem occurs because the Messenger service is disabled. The messenger service must run on the destination computer to enable the NET SEND command. By default, the Messenger service is disabled on computers that are running Microsoft Windows XP SP2.

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WORKAROUND

To work around this problem, change the Startup type of the Messenger service toAutomatic, and then start the Messenger service. To do this, follow these steps:

1. Open Windows Explorer.
2. In the left pane, right-click My Computer, and then click Manage.
3. In the Computer Management window, expand Services and Applications in the left pane, and then click Services.
4. In the right pane, double-click Messenger.
5. In the Messenger Properties (Local Computer) dialog box, click the General tab.
6. On the General tab, select Automatic from the Startup type list, and then clickApply.
7. Under Service status, click Start, and then click OK.