take a short look in the following document from rhyous, which describes a method to send messages to a client:
Instead of a pop-up message, you can open a URL...
Or, use Custom Data Forms if it's just a short notification... If you have the agent configured to show Custom Data Forms on Inventory Scan, you can schedule the forms and force an Inventory Scan.
Nice hear is that the result is put in the Database, so even if you give them only a 'close' or 'yes' option, you can see in the DB who actually had the message read. Even if they say later: "What message.....????" :-)