How i can check user availability without postbacking of page to server?

I have made a user signup form in asp.net. I want to check the user availability from user table made in sqlserver without postbacking of the page to the server. Checking user avai开发者_运维知识库lability when the focus out from textbox or during text change.

Please provide me code or source in vb language if possible.

From client-side, the only option is AJAX web services using JavaScript in the front end and an ASMX in the backend: http://dotnetslackers.com/columns/ajax/ASPNETAjaxWebService.aspx, or using the pagemethods approach: http://aspalliance.com/1922

HTH.