I just did something like this the other day using an UpdatePanel with an async postback trigger. There are several ways of achieving the same result but this method caused the least amount of grief and was pretty simple. The last link on this page should get you started.
You might also look into ASP.NET AJAX which I believe has a Timer control that can be integrated with an UpdatePanel. In fact, I’m pretty sure you can use System.Timers.Timer in .NET as-is… you’ll just run into problems when trying to do too much with it.