popup messages?

Hey all.
I have an interesting problem in the school where I work. One of our labs received a message saying "jump up and down". It said it came from computer "xxl123", which happens to be in a different workgroup, in a different lab. I had this happen to me the other day, but I'm not sure what to do to stop it. All the machines involved are Windows 2000 machines. The affected lab has Norton Antivirus running, and Clean slate. The "accused machine" is running Windows 2000, but I'm not sure what else, because that lab is taken care of by outside vendors. I haven't actually seen this happen where I work, I've only received complaints, one from a fellow IT worker. I have seen where the "General Protection Fault" box can be used to send messages, but I don't know how to block it. Any suggestions will be appreciated. :)

Sounds like the old Net Send command in WinNT.

Do all or any of the machines have software firewalls? You could try opening command prompt and doing a netstat /a to see which port the popup is using (when it appears) and block it if they do have firewalls.

Are there records of who uses which computer at any given time? You may be able to find out who's sending the messages.

If it's Windows messenger service and you don't need it for any other reason you could disable it.

Start>run, type services.msc then double click and disable "messenger" service.

Thanks for the quick replys. I'm betting that's what it is. :)