When you create an ASP.NET Web service, you choose a location of either File System or HTTP or FTP. If you choose File System, then the ASP.NET Development Server process is used. If you choose HTTP or FTP, the IIS process is used.
If your ASP.NET Web service is running in the IIS process, you can choose to run your unit test as a nondefault user for security. A nondefault user is a different process identity.
For example, if the ASP.NET Web service must access resources on the computer such as folders, files, or a database, you can choose to run it so that its permissions on these resources are exactly enough, and no more.
If the Web service requires permission that the nondefault user does not have, the unit test might fail. For example, the ASP.NET process that is running as a nondefault user might have restricted permissions, but still might try to generate temporary files to the %WINDIR%\Temp folder. This would fail if the default permissions on %WINDIR%\Temp do not allow the process sufficient access to generate those files. In this case, for unit test execution to succeed, you would have to grant the process higher permissions on the %WINDIR%\Temp folder.