IIS Express Web Server - (Every) Port is in Use?

Today I installed VS2010 SP1 Beta, and IIS 7 Express, as I currently have IIS5 on my local development machine and thought I'd try IIS Express out.

I have gone into the properties window for my Web Application, checked the 'Use IIS Express' checkbox, created the Virtual Directory when it has prompted me, but then get the message "Unable to launch the IIS Express Web Server. Port x is in use".

I have tried changing the port number from the default value that was in there, to numerous other port numbers, but whatever value is there, I am always getting this message, and am therefore unable to use the IIS Express Web Server.

Does anyone know what the problem might be, and how to get around it?