The user interface in Profiler for web applications leaves a lot to the imagination, so I'm sorry about that. Basically, the "profile on original port" option, as far as I can tell, does nothing but provoke an unnecessary restart of IIS. Port 80 will still be unavailable when IIS comes back up. I would not bother using this option and stick to "profile on unused port", which does not stop IIS.
If your web app absolutely needs port 80, then stop the individual website that listens on port 80 and use profile on unused port and specify 80.
If your web app is running in .NET Framework 4, there is the attach option, where you don't need to stop or restart anything.
Please note that none of this applies to Windows XP (IIS v5) but I assume from the problem description you must be using IIS 6+.
Also, we do not recommend using profiler in a production environment as it will consume additional system resources and disable recycling, leading to additional trouble on what is probably an already burdened web application.