Instant Messenger Script

Hi guys, I need some software making or altering if it already exists. Bascially what I need is to be able to install the software on mutipule PC's around the world and when installed I can simply type a message in at MY END and the message will be instantly displayed on each users PC's who has the software open on their computer in POP UP form. I need it customised so that the users CANNOT reply to messages... they can only read them. I also need it so that once it's installed on their PC's it will automatically open and run in the background when they start up their computer without the option of them disabling it (Unless they un-install of course) I need to be able to add my company logo to the POP UP that appears when ever I message them. Need this ASAP Regards Paul