MS AJAX Extensions is an assembly (DLL) that must be present on the target system in order for your AJAX-enabled ASP.NET application to work. To simplify deployment of AJAX-enabled ASP.NET applications, you can download
this open-source Visual Studio 2005 plug-in. It makes MS AJAX Extensions redistributable in the form of a Setup project prerequisite, just like SQL Server Express, for example. Including the ASP.NET AJAX setup in your setup project prevents your clients from having to find and download MS AJAX installation on their own.