Invoke Alert Globally after specific interval on any page

[MS News]

Helllo
I have Web Application in ASP.When I log in and ho to the home page then
"Alert Notification" for New Message comes.Now i want this notification to
come on every page where i am in application that is on any other page.If
new message come then this notification should come.
Note:I can not invoke this Alert function on every page for this i will have
to add this code in each page.
thanx
Usman