IIS Express is a lightweight, self-contained version of IIS optimized for developers. IIS Express provides the power of IIS7.x while making it easy to develop and test websites. Why we say that its has power of IIS7.x because it has the code base of IIS 7.5. Visual Studio 2010 SP1 Beta allows you to build and test web applications using IIS Express instead of the built-in ASP.NET Development Web Server (Cassini). If you want to use IIS Express with VS 2010, you have to install IIS Express separately on top of Service Pack 1
For any new web project or for a new Web Application we can converted to use IIS Express very simply by just right clicking on project and select “Use IIS Express”
For any existing project, you have to go to project properties to see the IIS Express Version. “Use IIS Express” won’t be available with existing web project.
But, if you want to use “IIS Express” for all of your web application, you to go to Tools > Option > Web Project and Select the Check Box “Use IIS Express for new file based web sites and projects”. This will set all of web project properties to run with “IIS Express”
Do you know, you can run IIS Express and Cassini ( ASP.NET Development Server ) Side by Side. Read the complete post from here