Starting with 2005, VS started this behavior of when starting debugging session it spawns up a webserver for every project in a solution. I have a solution with 15 projects
In Visual Studio 2008, there is an entry on the Properties page for the project called "Always Start When Debugging".
Note you have to get to this by selecting the project and going to the Properties pane (or right-clicking Properties). This option is not present when you double-click the project and open it in the main editing pane.
VS by default sets this value to on for all your web projects. Turning it off will solve this problem.
[editorial]This is fairly annoying and I wish the default were false![/editorial]