To make this change for one project at a time:

In  “Web.config”find the line saying “compilation debug=true” and switch this to false.

To make this change for the entire server:

In “%SystemRoot%\Microsoft.NET\Framework\%VersionNumber%\CONFIG\Machine.config” find the line saying “compilation debug=true” and switch this to false.

More info on this from Microsoft:http://support.microsoft.com/kb/815157