Winpopup uses Netbios (and Netbios over other protocols.)
In order to recieve the message, the user has to have winpopup.exe running, and also, you have to send it to the correct user or workgroup. In other words, you have to be able to see them in network neighborhood in order to send things to them. You can try finding their IP address and adding it to the lmhosts cache file, then running nbtstat -R to reload it, and then sending it to the computer name you specified in the cache file... I oughta try that.
