The following module was built either with optimizations enabled or without debug information

When you get the above error you can resolve it by the following way, Tools –> Options –> Debugging –> General –> Enable Just my code (Managed only) –> un check "Warn if no user code on launch" 

I encountered this problem in Visual Studio 2008 and like others here, my project settings did not have optimizations enabled and debugging information was configured. My problem turned out to be that the project assemblies stored in the GAC were from an older build. I think when I started debugging, VS tried to map between the GAC assemblies and the new source and could not do so and threw up this message. Deleting my old assemblies from the GAC resolved my issue.

Many a time while developing a web application you may come accross “Server application unavailable”

When you get this error in Windows XP make sure you have given ASPNET user sufficient rights on your physical application folder, it mainly requires read permission.

When you get this error in Windows 2003 server you can right click your virtual directory – permission – grant aspnet user sufficient rights there.

If the above solution doesn’t work here is another alternative,

You could try editing the Machine.config (located in
C:\WINDOWS\Microsoft.NET\Framework\v1.1.4322\CONFI G\).

Look for a key userName=”machine”. If you set it to “SYSTEM”
instead the aspnet_wp.exe runs with a different user (with
higher privileges).

Hope this helps