How to Give A popup msg in asp.?

Question

hi frnds..

 

      i have developing a web application  for a LAN Network. 

      Now my requirement is that i want to show a popup msg to a particular user or clients.

 

   For example......if the application are using by 10 users and i want to give a popup msg to a particular users.

   how it can... please lead me ......... 

 

THanks.......