In Visual Studio Orcas Beta 1, the AJAX ScriptModule is listed in the web.config template used for version 3.5 web projects. There is a known bug (DevDivBugs 78367) that prevents Visual Studio F5 debugging from working on IIS7 / Windows Vista when a module in the IIS/ASP.NET integrated pipeline has an event handler for PreSendRequestHeaders or PreSendRequestContent. Unfortunately, the ScriptModule listens to these.
There are two workarounds:
If you need to use the IIS/ASP.NET integrated pipeline and AJAX, you’re best option is to manually attach to w3wp.exe instead of using F5 debugging.
This issue is fixed post Orcas Beta 1.