To get you started again, I know you can remotely script like WMI scripts on clients.
You may want to look at running a script remotely that contains the warning you need, rather than using a service to send a message per se using Windows Messenger Service. Like have a critical error script that passes text from the command line, and have a local script that packages text for the command line and launches the script that runs remotely.
You will want to check out Appendix G in VBScript 2nd Edition Programmer's Reference. Maybe there is some code on the site. That chapter covers the Windows Script Host Object Model, and specifically I am looking at WshRemote. (I have to leave work right now so can't follow up till later.)