The aspnet_wp.exe executable (or w3wp.exe on .NET Server) implements the HTTP runtime, which controls all of the components that make up a Web service. This executable runs an ASP .NET worker process controlled by IIS. On a single-processor system, IIS 5.0 runs all Web applications, including Web services, within this single process.
The most important point to remember is that IIS 5.0 currently has the limitation that only one debugger can be attached to this ASP .NET worker process at any one time. Even though internally the worker process isolates your Web application from other applications by using application domains, it's not possible for more than one developer at a time to debug Web services (or other Web applications) on the same machine. The Microsoft development model seems to encourage each developer to test and debug her Web service on her local machine before moving the service to a proper Web server in a production environment. If you want multiple developers to debug their Web applications on a single machine using IIS 5.0, you're unfortunately out of luck.
This restriction goes away on IIS 6.0, because each AppDomain (logical process) can run in its own application pool if you so desire . Because each application pool is in turn running its own w3wp.exe process, multiple developers can debug on a single IIS 6.0 Web server.