Many times we face a requirement where we are asked to customize a dialog box in CRM, either with the help of JScript or HTML page. But Dialog are not available for client side extensions except custom assemblies. You cannot use JScript to Customize a dialog.
But there is always an UNSUPPORTED way:
You can customize the CRM 2011 Dialog page by customizing the page that run the dialog, the page is existing under the folder
$Microsoft Dynamics CRM Installation Folder$\CRMWeb\CS\dialog\rundialog.aspx , but it is not a recommended solution.
You can write some scripts to customize the dialog, but be careful when install a hot-fix that replace this page with default one, so keep a backup for this page on your solution after you customize it.