How to Start a Timer on button click in asp.net...& get alert window(pop up) after 2 hrs..?

Hi friends !
 
I have a requirement in my application where i will start a timer on button click....
the timer should remain running in the background and should give me alerts (pop up window) after 2 hrs and subsequently after 3,4,5 hrs...so that the user gets alert about the task he need to do after getting the alerts...
 
Anybody has idea about this ...please help... am eagerly waiting for someone to reply...
Thanks in advance..!
Good Day ..!