Best way to notify network users of network maintenance without relying on email

Hi I am wondering if anyone has any suggestions for the best way to notify computer network users of planned unscheduled maintenace. unscheduled reboots etc. I maintain a network that is set in multiple states and is in use about 20 hours a day via the users. I sometimes run into situations where it would be handy to send a message or alert via the network to the users and not have to count on them reading their email to receive the alert. I am familiar with messenger service but I also know that there are pitfalls with using messenger so I am hoping that someone might be able to recommend another solution or perhaps a way to use the Messenger service but in a way that is secure and protects the users from junk messages etc. The network is a Windows 2003 server based network with XP Pro/Vista Users

 Any help would be appreciated.

Thanks!!