Client modal dialog in asp.net with server-side code. (How should I do it)

I need a technique to work with what seems pretty simple!

I have a form with some server side logic to test. if the server side code indicates a problem, I want to display a modal popup for the client.

I'm having trouble getting it to work this way.

I found that if I keep the entire logical client side, I can use the javascript function

window.showModalDialog("url","title", "style");

However, the logic is currently on the server side. this is an ASP.NET 3.5 web project - is there any way to do this using AJAX controls like scriptwriter, update panel, etc.

Thank!