Messaging remote desktop users

Hi,
Is there a way to message users logged on to a TS machine to inform them of restarts, etc? I’ve tried ‘net send’ in the command prompt, but nothing seems to happen. Also tried using RealPopUp but this is limited to one session per machine (rather than per user) so is useless.

The TS machine is running MS Server 2003 Standard. Our main server runs SBS2003 (in case that’s relevant too).

User log in using an IP address as they are not connected directly to the network.

Thanks for any help!
Rich