In Visual Studio (2005, 2008 & 2010), if you have multiple web projects in a solution a separate instance of the development web server “Cassini” is launched for each web project when debugging. If you are only debugging a single web project all these instances are probably unnecessary. Setting a single startup project in the solution properties menu does not solve this issue, instead you have to explicitly stop each web project from launching a server instance. This is done through the properties grid in the properties window (View -> Properties Window) for the web project not through the Properties menu (right click on web project -> properties) (someone please give me a good reason why there are 2 properties menus).
In the properties grid for the project you want to exclude, simply change the “Always Start When Debugging” property to False. This prevents a server instance being created for that web project, rinse and repeat for other projects you don’t care about.