The Validation controls make use of a JavaScript script library that is automatically installed on your server when you install the .NET framework. This library is located in a file named WebUIValidation.js.
By default, WebUIValidation.js is installed in a directory named aspnet_client located beneath your Web server's wwwroot directory. If you change the location of your root directory, you need to copy the aspnet_client directory to the new directory; otherwise, the validation script will not work. If WebUIValidation.js can't be found, you receive the error Warning! Unable to find script library 'WebUIValidation.js'
Microsoft includes a command-line tool with the ASP.NET Framework named aspnet_regiis that you can use to automatically install and uninstall the script library. To install the script library execute aspnet_regiis -c, to uninstall the library execute aspnet_regiis -e. The aspnet_regiis tool is located in your \WINNT\Microsoft.NET\Framework\[version]\directory.
初学者经常碰到的错误，解决起来很简单。
摘自《asp.net unleashed》
